I've been to both (Longleat CP) and I preferred Bluestone. It's much quieter and more like a village than CP, the pool is far nicer (although smaller), the restaurants aren't chains and you aren't so 'trapped' - it's far easier to get back to your car and explore the local area. I didn't use the spa in either (pesky kids ) so I can't comment on that.
Have also been to both and agree they are pretty kid centric. However, if I was choosing btwn the two for an adult break I'd probably do Bluestone. It's generally a more relaxed atmosphere than CP's, the 'posh' restaurant is nice and you can get off site easily if you wish with plenty of lovely pubs etc around that area.
I didn't go to the spa at Bluestone but it looked nice so if I was there without kids, I'd give that a go.