The Merriam-Webster Dictionary of English Usage (MDEU) suggests that to is the ordinary word used to construct comparisons using prefer : “when it is used to compare two things in the same sentence, the second […] is usually introduced by to .”
They note that over (“Nine out of ten dentists prefer Crest over the competitors”), and rather than (“He prefers to stand rather than to sit”) are also used.
Rather than is especially helpful when the compared items are infinitive clauses beginning with to , therefore avoiding the problem of too many to s
( * “He prefers to stand to to sit”).
Above can also be used (“Prefers this brand above all others”), although they note that above , along with before , were noted by the OED as being used formerly.
MDEU notes that some commenters have criticized constructions with than and rather than , and suggests ultimately that plain than (“He would have preferred to fast than carry it”) is awkward because it is unfamiliar.
With respect to colloquial speech, I think that in some informal registers you wouldn’t use the verb prefer at all (which is a formal word), and would use like and a compatible syntactic comparative construction: “I like sleeping more than working”
english.stackexchange.com/users/39/nohat
There may be a transatlantic divide here.
As a speaker of BrEng I would always say I would prefer tea to coffee , and never I would prefer tea over coffee .
english.stackexchange.com/users/12952/Barrie England
A:
Would you like a ride?
B:
No thanks, I prefer to walk.
You can't use prefer over in this case.
Over is used when there are two clear choices in the phrase.
Think of over as setting a list of preferences and putting one over top the other.
I prefer jogging over running and walking.
I prefer fish over beef and chicken.
In these two examples, the meaning could be slightly ambiguous and sound like you're comparing two things, not three.
I prefer jogging to running and walking.
I prefer fish to beef and chicken.
english.stackexchange.com/users/134/OneProton
My brain does not differentiate the two:
"I prefer sleeping to working" "
I prefer sleeping over working
" Sounds the same to me- perhaps "prefer/to" flows slightly better.
On the other hand "prefer/to" is more likely to be ambiguous: "I prefer sleep to work"
In conversation the word "rather" seems more common than "prefer" but others may have different experience.
Either way- I would be shocked if anyone criticized your usage of "prefer/to" versus "prefer/over" in any unambiguous context- formal or not.
english.stackexchange.com/users/645/Sobachatina
They are both grammatically correct.
"Tea over Coffee" sounds the slightest bit metaphorical to me, and so might have some rhetorical implications in a longer narrative, but that's about the only real difference.
english.stackexchange.com/users/4972/Mitch
When we say prefer there is an implied over .
Thus, prefer over is tautological.
I think prefer to is preferable.
english.stackexchange.com/users/19099/Refwise
Many say that "prefer X to Y" has a more formal ring to it than "prefer X over Y". Are there any dialects where you wouldn't use "prefer X to Y" in colloquial speech at all? Conversely, are there any manuals of style that discourage using "prefer X over Y" in formal writing?
Which is correct: "prefer X to Y" or "prefer X over Y"?
english.stackexchange.com
<word-choice><prepositions><formality>
english.stackexchange.com/questions/1752
english.stackexchange.com/users/300/RegDwigнt
There is no agreement as to whether "prefer X over Y" is correct or not. Some will claim that the word "over" is unnecessary whilst others will claim that "over" and "to" are acceptable. Usage may differ between British and American English, however, "to" is probably more common in both. Both forms are correct but "prefer to" is more common, although in colloquial speech a "more than" construction is more usual.
There is no agreement as to whether "prefer X over Y" is correct or not. Some will claim that the word "over" is unnecessary whilst others will claim that "over" and "to" are acceptable. Usage may differ between British and American English, however, "to" is probably more common in both. Some would argue they are both grammatically correct, but the  Merriam-Webster Dictionary of English Usage suggests  that  'to'  is the ordinary word used to construct comparisons using  prefer. In BrEng you would  say prefer tea to coffee , and never  I would prefer tea over coffee .
They are both grammatically correct.   British English speakers may be more likely to say "I would  prefer tea to coffee". The  Merriam-Webster Dictionary of English Usage (MDEU) suggests  that  "to"  is the ordinary word used to construct comparisons using  prefer : “when it is used to compare two things in the same sentence, the second […] is usually introduced by  to .” There may be a transatlantic divide here. It appear that there may not be a meaningful difference between the two and if there is, it may we regional. However, seems to be a preference for using "to" with the verb "prefer". In any case, using "prefer" in informal speech might not be very common.
Both forms are correct but "prefer to" is more common although in colloquial speech a "more than" construction is more usual. There may be a US-UK difference here. In conversation the word "rather" seems more common than "prefer". They are both grammatically correct.  When we say prefer there is an implied "over" so it is tautological. 
The  Merriam-Webster Dictionary of English Usage suggests  that  'to'  is the ordinary word used to construct comparisons using  prefer. In BrEng you would  say prefer tea to coffee , and never  I would prefer tea over coffee . They are both grammatically correct.   
Some people would say that there is no meaningful difference between the two. It appears that "to" is the more common preposition used with the verb "prefer". Over is sometimes used when there are two clear choices in the phrase. The difference in usage might be regional. Prefer might not be used much informal speech in any case.