The number of rounds, by itself, doesn't determine the security of a cipher. You need to take the round function into account before the number of rounds means anything.
Nonetheless, I'd agree that there's a pretty decent chance that Serpent is more secure than AES. There are attacks currently known against AES that reduce the complexity by a factor of approximately 4 compared to a pure brute-force attack.
Cryptographers count that as a successful attack--but from a practical viewpoint, it's of precisely zero consequence. Even if you restrict yourself to AES-128, it's basically reducing complexity from 16 times the estimated life of the universe to only 4 times the estimated life of the universe (I'm sort of making up numbers here, but you get the general idea). With AES-256, the number is so much larger the factor of four shrinks to a new level of utterly meaningless insignificance.
Until/unless a dramatically better attack is found, real security is completely unaffected. In essentially every case, the problems you need to deal with and worry about are in how the cipher is used, how keys are generated, stored, and exchanged, etc. Changing from AES to Serpent (or Mars, Twofish, etc.) is extremely unlikely to improve your security (or anybody else's).
I should probably add: I'm probably as strong an advocate as anybody of having more cipher algorithms available and standardized. If you do a little looking, you can find where I'm cited in the papers submitted to NIST during AES standardization on that subject, giving use cases where including more than one algorithm in the standard would have been useful. Nonetheless, I have to admit that no (publicly known) current attack even comes close to giving a real reason to choose a different cipher algorithm.
