I've been to both, but both times was without my kids.
Aruba is much warmer, but tends to be much windier, IMHO.
Both beaches were great. The pool (if that is important) in St. Maarten, while pretty, was FREEZING cold...not good for a toddler!
The rooms at St. Maarten were huge, and IMHO would be preferable for a toddler...either place would be good, it's just a matter of preference and proximity!