#1 for me to. I actually prefer one of each, I like brown eyes to, but I prefer the eyes to, at the very least have eye liner, but surounded by color is even better. Of my current two horses, my horse has one of each, my only complaint is about her brown eye, it has to much white showing and it makes her look ill tempered. My other horse, a mini, has two blue eyes, w/ a completely white face and no eye liner. Other then those two my family's other horse also has one of each (and his brown eye is much nicer then my girls), and the pony I just sold had a partial blue eye.
Honestly the eye color would be the last thing I'd worry about when shopping for a horse, but I do consider it a bonus when the horse has a blue eye!
And just FYI, I've never found blue eyes to have more problems, it's the pink eye lids and white faces that just happen to usually also come w/ blue eyes that tend to have more problems, nothing to do w/ the eye color. And those white faced, pink eye lids could just as easily have brown eyes. In fact my spotted mule has a competely white face and brown eyes, and needs a fly mask to protect her sinsitive pink skin.