Both are lovely but I'd say Skiathos has more to do and is a bit livelier.
We found Lassi very quiet, everyone went out at 6pm and came back at 9pm so we were the only ones sat in the tavernas.
Lots of couples with very young children, prams and pushchairs in Lassi so it would depend on their ages.
You couldn't go wrong with either of these islands in my eyes.