We have both and use both communally. If you live together why have one be "hers" and one be "his?"
In our case, the big one is an ipad 1, too old to run some apps, but more comfortable for most games (DD is on a as Plants vs Zombies kick).
The little one is the one we grab to do games that won't work on the big one, or if the big one is taken.
I don't think smaller is better, unless it's a "it fits into my purse" thing.