Hmmm, I've played CoH, and am now playing L2, so I'll chip in on this one.
CoH is a lot of fun. It's kinda like somebody took Diablo2's gameplay and made it into a super hero game. Lots and lots of fast-paced action. Well, depending on your archetype and if you're solo or group. Overall a fun game, and if you can resist the urge to keep making new heros you'll get to see a lot of neat enemies. Otherwise you'll be fighting street thugs for weeks
Downside to me: Nothing *but* beating on happless villains. Great game though if you only have little chunks of 30 minutes to an hour to play.
L2 is even less interesting on the PvE side of things. More monster variety, but they're even dumber and there's less story "fluff" to read. But, full PvP! Even CoV(illians) will not have that. And castle seiges! From seiges in Shadowbane, I can already tell that sort of thing is a total blast (for me). At least, long as the other side shows up to fight anyway. And with only 5 castles to fight over, I'd say they will in L2. A lot. Downside? Takes a while to get there in levels. Bigger overall time commitment on this one.
Which is more for you? <shrug> no idea. But that's the games in a nutshell from my experience. Hope it helps