RE 5 is as solid as a game can get. In my opinion, it's more of a keeper because it has a full offline component whereas L4D's strengths lie in it's online component which you may or may not always have access to.
As for the L4D characters, I really do miss the old ones. I hope that they can at least make a cameo but I understand the decision to use new characters. Afterall, L4D2 is a new game and if they used the same characters, it would seem like less of a sequel and more of a map-pack. Who knows, they may end up overhauling the characters like they did with the original L4D!

Either way I can't wait for it.