First, I'll go over the pros and cons of Lilligant and Serperior.
Gets Giga Drain, Sleep Powder, and Quiver Dance (some of its best moves) before level 30.
Great set-up sweeper with Sleep Powder and Quiver Dance.
Great base 110 special attack, and a decent speed stat of 90.
Powerful late game STAB with Petal Dance (with no drawbacks if Own Tempo)
Gaurenteed great nature in Timid if in Black 2.
Requires Sun Stone to evolve. (Earliest is available in Nimbasa City)
Pretty much only gets grass type moves.
Available later than Snivy, in Castelia City in White 2 or on Route 4 (if you trade a Cottonee) in Black 2.
Worse defenses than Serperior.
Available immediately, as a Snivy.
Better defenses than Lilligant, with 95 in both.
Has a slightly better movepool than Lilligant, learning some Dragon type attacks by move tutor.
Pretty fast, with a 113 base speed stat.
Is a decent mixed attacker, due to its identical attacking stats.
Mediocre attacking stats, with 75 attack and special attack.
Gets best set-up move, Coil, at level 38 as a Serperior. (ten levels later than Lilligant)
Main STAB is Leaf Blade, which it gets at level 32 and remains its most powerful move without drawbacks until you teach it Energy Ball by TM, but is still less power than Lilligants Petal Dance.
With that being said, I would say Lilligant is better. It has a very powerful STAB attack with Petal Dance (combo-ing well with Own Tempo) and gets some of its best moves (Sleep Powder, Quiver Dance) much earlier than Serperior. Even though it's available later than Snivy (and it's kind of a pain to evolve), neither of them are very useful against the second and third gyms (being Poison and Bug).