Definitely go for Enclave. It's one of Starbreeze's finest games. I really wish they made Enclave 2, which was actually planned at the time, but well, still no word from it.
On the other hand, Advent Rising looked interesting to me before it was released, but after I tried out, it turned out to be a poor Halo copy. Couldn't play for more than thirty minutes. Decided to try it out once again after a year, still a bad game, didn't stay for long.