Hard to say, both are pretty damn hot. Mikuru has the bigger "Assets" although Haruhi is also really curvy and also I was surprised at how big her "Assets" were when she was singing in that bunny outfit.
I would say though that if Haruhi had long hair like this:
Suzumiya Haruhi, hands down. There is no competition. I like redheads, but I am a sucker for brunettes. Haruhi's voice is sexier too (in Japanese and in English).
But really, it's a matter of personality. Haruhi is bossy and assertive, which is sexy. She is also a deep thinker. She is extremely athletic and intelligent. However, the best thing about her is she has a free and independent spirit. She does what she wants, and she is capable in anything she does. A strong, independent woman is truly attractive.
Asahina Mikuru's submissive personality is sexy in it's own way, but it just doesn't beat Haruhi's dominant personality. Of course, we all know Kyon isn't completely submissive to her... so that says something about both of them.