Only ever been to Whinfell.
Like someone said previously, it depends where you are. I wouldn't waste a day travelling because you can use the facilities from 10am on your arrival day until you want to eave on departure day!
We went last year with our 4 year old DS and 20 month old DD. Going again for Bonfire weekend but 9 of us in total!
My 4 year old LOVED the pool.
Whinfell has two small slides in the children's section plus the splash area at the top. There is a wave pool, 'lazy' river that leads into the outdoor area where the wild rapids ride ends. There are two tube slides plus a large white slide into it's own lagoon. There is also an activity pool too.
The parks are good but the one by the Sports Plaza is more toddler friendly. We stayed at Bilberry Wood which is directly opposite the big park plus the center. The park is adventurous but not toddler friendly IMO.
Lots to do and see (all very much the same at all the resorts). You can google maps for the swimming areas (and youtbe videos clips are good too) to get an idea as to what the swimming pools are like.
We based our decision on: price, location and size. Whinfell tops it for us although slightly further for us than Sherwood. All my friends go to Sherwood except for one family who say they prefer Whinfell. We tried to do Sherwood but it was going to be £400 MORE for the same time and same lodge.
No brainer.