Ah Cookie Clicker's got fancier graphics (although they're kinda heavy, I'd turn them off) and it goes... eldritch at some point. It's definitely the longer of the two to play, I recall finishing Paperclips in about three slow workdays.

There's one, I don't think Cookie Clicker has an ending. It's got countless multipliers, but eventually you're completely out of upgrades and achievements and it just sort of peters out in diminishing returns.

One remark though: if you've been playing for a while, save & back-up your save games. It stores progress in a cookie (I think) and they have a tendency to expire / get lost after a while. I just opened it up (it seems I last played a year ago) and it started a new game, but I had it backed up. Save before you stop playing.