Depends on what you're looking for. Jayce is the best champion in the game right now and pretty much beats anyone in lane but he's going to be nerfed next patch. I bought Lissandra yesterday and am having a lot of fun with her. She's really good against melee/short ranged mid lane champions and her ult can be absolutely devastating in team fights.
Both do lots of damage, both have major teamfight presence. Hard decision but with Jayce getting nerfed next patch I'd probably go with Lissandra.
"You don't need a reason to help people." - Zidane Tribal, Final Fantasy IX
Jayce is getting nerfed and he has a very high skill cap, but he's strong and has a lot of utility.
Lissandra is not getting nerfed AFAIK and doesn't have nearly as high a skill cap. Also has a lot of utility (and far more CC).
Jayce is ideal in poke comps, which not a lot of solo queue players know how to run properly (they just go all in before you and Nidalee have landed a single poke lol)
CC wins games, even when your team is inept, because of its power to disengage or initiate and draw a group of morons together into a singular goal: win a team fight. This is why champs like amumu and malphite are banned so often.