Hmmm, have to say Brands, It's just got everything, history, exciting curcuit, challenge. Just unforgettable. but I am biased.
Thruxton may be easier for your first, as there's quite alot to get used to at Brands to get the best of it, but they are very helpful so if I had the choice gotta be Brands.
PS get track day insurance if you are using your own car....the old bill will never believe you pranged it just outside the circuit..