We've been to both.
Elveden is flatter than Longleat, so easier if you're on bikes. I recall it being smaller than Longleat but of course that makes it easier to walk into the center where the swimming stuff is. Personally I found both pools just fine in terms of fun, but we spent less time overall in Elveden (3 days vs 4 or 5 in Longleat). Possibly the other factor you might consider is going off-site at Longleat to the safari park if you're so inclined.
Hope you have fun whatever you decide!