I like it when the two mix. Mostly in cute nerdy girls with great bodies. UGGGG. So not fair.
If I had to choose however I'd go with cute. Cute has something charming about it that never really seems to go away, and for me mostly resides in the smile and eyes, where as sexiness can 1) be synthesized (fucking modern bras... you never know what you're getting) and 2) fades with time.
So yeah, cute girls all the way. Still though I'll always drop everything for a cute nerdy girl with a sexy body.
Oh, one caveat: Sexy can also be in the actions of a girl, they can say things or do things in a very hot way... though sadly this seems to be a trend giving way to the awkward generation as I've only really found it in older women. Anyone can be sexy if they just act the right way, in fact that how you end up seeing ugly guys with hot girls... Girls are just amusingly less superficial.