I would say coton would be the better choice for MANY reasons.
Bichons are very over bred and unless you find a really great breeder then they can have many health AND temperment problems. They are fun little dogs but not really easy to find a good one.
Cotons are rarer and maybe a little more expensive but not by much if you got a good bichon. Plus if you got a not so great bichon for some reason the medical problems could equal to way more.
In either breed you want to see all health tests done (ofa, cerf, thyroid, and any others recommended by the breed club).
Cotons are fun little dogs. I dont have one but a few people here do. Very friendly and playful dogs.