As late as 1777, when the Royal Standard Dictionary was published, the predominant pronunciations of “either” and “neither” in England were “ee-ther” and “nee-ther.” But that gradually changed. In its 1907 edition, The Oxford Dictionary remarked that “eye-ther” was more prevalent in the “educated speech” of Londoners. H.W. Fowler predicted in 1926 that this pronunciation would “probably prevail,” and by 1965, when Sir Ernest Gowers revised Fowler’s A Dictionary of Modern English Usage, it had “almost wholly displaced” the long e pronunciation.
For a long time, the United States stuck with “ee-ther,” for the most part. In 1873, the philologist W.D. Whitney harrumphed that the ”eye-ther” pronunciation had “spread …by a kind of reasonless and senseless infection, which can only be condemned and ought to be stoutly opposed and put down.” A 1928 satirical sketch called “The Lady Buyer” noted of that personage,
always, standing her in good stead, and ready at the tip of her tongue is her crystal-clear, British pronunciation of “either.” She says the staunch word with such hauteur as to make one forget other mistakes and even feel apologetic for having noticed them. Nothing on earth could make her whisper “ether” in the darkest corner of a stock-room. She knows it would ruin her socially.
Memorably, “eye-ther” was one of the British pronunciation choices (along with “to-mah-to”) in Ira Gershwin’s 1937 lyric to “Let’s Call the Whole Thing Off.” In 1961, Hans Kurath and Raven McDavid called “eye-ther” “a sporadic feature of the cultivated speech of Metropolitan New York and Philadelphia…. it is in all probability a recent adoption from British English.”
My ears tell me is that “eye-ther” and “nIe-ther” are currently on the rise in America, especially among young people. My own millennial-generation daughter, despite having two parents who say “ee-ther,” says “eye-ther.” On the pronunciation site Youglish, six of the first twenty American utterances of the word are “eye-ther” (all youngish people), and fourteen are “ee-ther,” including Barack Obama and Hillary Clinton.
I would be interest in the observations of readers, both American and British–or any other English-speaking country, for that matter.
notoneoffbritishisms.com 
52yo Brit here. I think this is one of those fairly rare cases where I use the two different pronunciations more or less interchangeably, though perhaps with a slight tendency to default towards “eye-ther”. FWIW, I was and raised in Yorkshire (which makes me a northerner) but have lived the greater part of my life in the Midlands.
“Eye-ther” and “nie-ther” have always seemed to me – an American – as what Edwin Newman once called “putting on the dog,” or pretentious (like “prevenTAtive.”) Young people are often pretentious, especially when there’s an opportunity to contradict elders.
I once heard a young American performer give a reading of Dickens’ “Christmas Carol,” and besides using “to-mah-to,” he also insisted on using “po-tah-to”! When I pointed out the discrepancy, his defense was that he was being consistent. (As if the English language has ever been consistent!)
Now, how about taking on “en-ve-lope” v. “ahn-ve-lope”?
sorry, what wrong with preventative?
Both Edwin Newman (and I) consider(ed) the internal “ta” in “preventative” unnecessary. As I recall, he made a bit of a joke by adding the “ta” to other words of similar construction, even creating a string of “ta”s, something like “preventatatatative.”
52-year old American of New York upbringing, mixed Southern and Mid-Western extraction. I’m a lot like Rob, I tend to slide between the pronunciations. I think I’d be more likely to use “eether” in a less formal setting.
5*3* year old Australian here. What those pommy and yank youngsters are saying is pretty much how it is here, too. Either pronunciation, usually depending on what the words either side are.
Or maybe it’s the place in the sentence… since in my head the previous sentence is ‘E-ther pronunciation, usually depending on what the words I-ther side are”.
I’ve lived in both Canada (British Columbia) and the U.S. (Los Angeles) for 50 years, switching accent, pronunciation, and spelling accordingly. Canadians say eye-ther, in my experience, whereas in the U.S. folks say ee-ther.
As a Canadian in my 60s, I have always said “eye-ther” and “nye-ther” and so do my children so it’s not a generational thing. We also all say “leh-shure” rather than “lee-shure.” And we say “deh-cal” rather than “deckle” or “dee-cal”, one of those rare times when we speak independently of the US or the UK.
I think you are correct in hearing an uptick in the EYE pronunciation of both words in the US. I am an EEther/NEEther guy and don’t recall encountering the other pronunciation much in my youth (60s and 70s). By the 80s, I had a few friends who were EYEther/NYEther folks, but I considered it a regionalism or family thing. My own opinion as to the reason for/source of the change is the usual uncharitable one: misguided Americans somehow thinking British is in some way preferable, rather than just different.
Late sixties Brit here and like others have said, I seem to use both pronunciations, but it seems to be entirely subconscious which one I use.
Sixties Brit here. I tend to say eyether, having had it drummed into me as a child that eether was “common” and should therefore be avoided. But increasingly I use both pronunciations, and have noticed that others who I might expect to have a similar background as me do the same. So in some circles in the UK at least, the EE pronunciation is gaining ground, I believe.
When Queen Victoria was asked whether it was ee-ther or eye-ther, she replied, “Eye-ther will do’.
Here is a reference. Either is the German pronunciation. Queen Victoria, as well as Prince Albert, spoke with a German accent. https://forum.thefreedictionary.com/postst163457_How-should-I-pronounce–either–.aspx
23-year-old American here. I say “EE-ther” and “NEE-ther” as do most older people I know, but I’d frankly suspect that “EYE-ther” and “NYE-ther” are predominant among others my age who I interact with.
Here is a reference. Either is the German pronunciation. Queen Victoria, as well as Prince Albert, spoke with a German accent. https://forum.thefreedictionary.com/postst163457_How-should-I-pronounce–either–.aspx
My immediate instinct was that this smacks of urban legend. Sure enough: as one of the commenters in that thread pointed out, “The ‘eye’ pronunciation is noted in John Jones’s Practical Phonography, which was published in 1701, 118 years before Prince Albert was born.”
I think both pronunciations would have existed pre-Victoria and Albert. However, they could have made the eye-ther pronunciation the posher one. As a subsequent post says, “Well, that’s the story reported by one Edward M. Perrin in a 1980 book entitled On Language by William Safire, who was an expert on “good” English for many years in The New York Times.”
In my experience as a 66-year-old Englishman, the two pronunciations of either and neither are in free variation in educated speech in England. Running them over my tongue and thinking about it a little I suppose I use the ‘eye’ variants more often; and the ‘ee’ variants seem slightly more effortful and ‘marked’ in the linguistic sense. But I don’t think anyone ever gets criticised here for using one or tother.