I find cockatiels are much more forgiving, more solid and interactive compared to the parrotlets.
Parrotlets (or at least the handful I've met) are very sweet but seem to be more of a bully (I think it's to make up for their petite size
BUT you never know. Your best bet, in my opinion, would be to visit the potential birds and interact with them. You could be set on a 'tiel yet have your heart stolen by the antics of a P'let





