Well, Lilligant gets access to quiver dance and sleep powder in which the former is one of the best set up move in the game while the latter is a great status inflicter. Also, it has access to giga drain for reliable recovery or you can go for leech seed, and a good coverage hidden power will round it's set nicely. On the other hand, serperior gets contrary leaf storm, which raises it's special attack by 2 every time you use it which makes it a good wall breaker. You must have hidden power on it or else it gets walled by many types, and serperior is actually pretty fast.
In the end, both choices are solid grass types so just pick what you like better.