1 arena is better than the other, the warm up is a nightmare. There is a footpath that runs round the edge of one grass arena. I had on one occassion people walk round the edge of the arena whilst i was jumping.
They water well most of the time, they use the one that lizzie_liz mentioned as their main ring, they usually run just clear round type stuff in the other one unless its a big show. The warm up isnt great but I think they had tried to improve it by putting down wood chip.
The good thing about blewbury is they dont build very big... unless they let Mr Ashworth build....
Warm up is a nightmare and i'm not fond of the indoor arena - i've only jumped there once in the winter but it was very dark and dingy. In the summer it's probably nicer though