i've never been to PPW, but it's on my list to go this year but i took my 3 yr old dd to legoland last yr she had just turned 2, was just right for her as there was lots to do, there was a pirate show and duplo land to play in. plus i think there are some new things opening this year. definitely a dry day though otherwise makes a dull day if ya get wet, and maybe a weekday (if possible).
legoland has a water play bit so i would recommend spare clothes, coz if it's not chilly she might wanna go play. but i think it depends on the traveling time as to which you go to.
I hate legoland. It's nothing more than a giant fairground with queues for an hour. I would wait for your local fair to come round and go there - no queues, £2 per ride instead of silly money for the whole family.
Haven't been to PPW but I hear much better things about it.
Duplo land is being redeveloped at the moment and re-opens in May. The water splash bit is being upgraded and looks great, my 4 year old loved the old one (on a hot day!) Plenty to do for a 4 yo at either park, but definitely need dry weather if we get any.