I hear "dayta" more often, but what's the correct pronunciation?
There's no such thing as "correct" pronunciation.
Now, to answer your question, here's what LPD3 says on this (Wells 2008):
NB: ț stands for the (voiced) alveolar tap (flap) here. Wells uses a slightly different symbol, not the usual IPA one.
A historical perspective:
The eleventh edition of Everyman's Pronouncing Dictionary by Daniel Jones (the 1960 reprint) gives ˈdeitə as the primary variant for British English, whereas dɑːtə is given in brackets, which means, in Jones' notation, the less frequent form that is still in current use.
I vaguely remember being told that day-ta was the correct pronunciation, and that was because there was a vowel (the second 'a') following the consonant (the 't'). Now whether or not that actually applies, someone please comment because I'd love to know if I was given a load of hogwash there.
Personal experience, I find that I hear and use day-ta more than dah-ta. It is similar to the different ways that 'SQL' is pronounced among us techies - some spell it out, and some pronounce it as "sequel". Thinking about it, I find that when it is the word "data" all by itself, I use day-ta, but when it is part of another word ("database", for example) I tend to use dah-ta instead. Again, not sure if this is something that I just happened to have picked up over the years or if it is even correct. Anyone else notice the different pronunciation in situations like this?
This question reminds me of the film Gravity. Watch the first five minutes here. On 1:55 you can hear "Houston" (Ed Harris) saying "day-ta"; on 4:47 Dr. Stone (Sandra Bullock) says "Dah-ta", only to be answered "we are not receiving any 'day-ta'".
Now, Ed Harris was born in New Jersey and Sandra Bullock is from Virginia, but raised in Nuremberg. It seems that the correct pronunciation of "data" is a matter of geography and not so much of grammar.
Well, I know this is not the most comprehensive study, but I wanted to share an interesting case of 'day-ta' vs. 'dah-ta'.
It's DAY-ta on Star Trek, which is perhaps the ultimate reference. :-) I cannot imagine Patrick Stewart saying "Dah-ta."
P. S. An editorial reminder (as mentioned earlier) that data is plural; e.g., "the data are clear" not "the data is clear." Feel free to edit this answer for references to data being either plural or singular in usage. Let's keep it humorous, if possible.