I just got back from from a weekend at the Sherwood one today, so I was quite interested in this thread!
I have been to Whinfell and Sherwood and I would definately vote Whinfell! I have been there loads and I love it. The lodges are lovely and the free pool is HUGE with loads of little pools and whirlpools to go exploring. The main centre is all under one roof which is handy if the weather is inclement. There is also entertainment on an evening if that is your type of thing (you have to pay though!!)
Sherwood is nice too, but the main centre is smallish and outdoors. Not really too much of an issue, but it was a pain this weekend when we had to dress our baby up in millions of layers after swimming just to go over the way to the restaurant and undress her again! We stayed in a comfort lodge this time (the cheapest ones) which were very nice considering - but the bedrooms got very very cold. At one point it was 12 degrees in our room!!
I think which ever parc you go to, it is worth paying a little bit more and getting at least a comfort plus lodge or better because the new decor is really nice and you get nice perks like a dishwasher, dvd player and fluffy towels!
Have a fab time wherever you go!!!