As far as stats go, the one that's better depends on what you want. However, Sceptile clearly wins in terms of moves.
Serperior is more defensive with 95 for both base sp defense and regular defense. It's base attack and Sp attack are both 75, giving flexability as to what kind of moves apply and making it a mixed attacker. However, it has little variation in move type. It can learn Areial Ace, Dragon Tail, and Wring Out as non-grass moves, plus many grass moves including leaf blade, giga drain, and leaf storm. As for stat boosters, it has coil (boosts acuracy, defense, and attack) and growth (boosts sp attack and attack).
Sceptile is much more offensive. It's base attack is 85 and base sp attack is 105. However, it has a defense of 60 and a sp. defense of 85. It also is faster. With moves, many of them are physical rather than that great special attack, but hey, they still beat Serperior's 75 attacking stats. Among the notables are Earthquake, X-Scissor, Dragon Claw, Arial Ace, focus blast, crunch (via breeding), night slash, iron tail, and brick break (sadly, all physical). For grass moves, it, like serperior, can learn leaf blade, giga drain, and leaf storm.
Personally, I prefer Sceptile because of the greater freedom in moves and higher attack.
