You really can't go wrong with rosys, imo.
I haven't found the size difference to be significant at all when it comes to care, but girls do tend to be a little bigger.
Personally, if I were getting a rosy for a pet I'd choose a female. Females take longer to get to sexual maturity... if down the road you decide to breed your pet it will be much easier to pair up a girl.
Something else to think about... When choosing a rosy, the biggest thing I would look for is that it be locality specific (or a good morph).
My webpage (not sure if I'm allowed to post a link) has over 700 images of rosys from more than 100 localities.
As I said, you can't go wrong with rosys. They're awesome!
-Josh
