I have been to both too (Sherwood for Center Parcs). I would by far recommend Bluestone.
Center Parcs is fine, but you are charged for everything - I always remember seeing the activities list and where it said 'nature trail' it said 'this activity is free'!! So it bloody should be!
We went to Bluestone last July with DD (who was nearly 2 at the time) and my DSS and his friend (late teens) and had a fab time. The accomodation was superb - just about the best self catering we have stayed in, at least in this country. We stayed in a Ramsey lodge - 2 bed, 2 bath. The bedrooms were downstairs and upstairs was the open plan living room/kitchen. Great cooking facilities, really comfy sofas, plenty of room for all 5 of us (including the hulking teens!). My only complaint was that there was no washing facilities (a bit daft for a family-centric place).
It still needs to bed in a little - it's still very new looking (akin to a new golf course say), and the plants/trees need time to mature. The whole site is very hilly, so make sure you book golf buggy/bikes as soon as you're able. I didn't realise I was pregnant when we were there and wondered why I was getting so out of puff walking up to the pool!!
The pool is fine, and would be great fun for older children. Try to go outside of school hours as it does get very busy. It's not the sort of pool where you can have a proper swim though, more of a splash.
The restaurants on site are fairly good, but need improvement. The food was generally ok - good, but service was pretty slow. Fireworks over the lake on some evenings are lovely.
One of the best differences between Bluestone & Center Parcs is that at Bluestone you are actively encouraged to go out and explore the stunning Pembrokeshire countryside (rather than not seeing outside of Center Parcs for however long you are there for). There is so much in the local area...Folly Farm for small children, Oakwood (which you can see/hear from Bluestone) for the older ones!
We didn't do any of the activities when we were there - just went out & about and generally chilled. We would definitely go back, in fact we may well go this Sept when new baby will be about 6 months old