I'm running Mint 15 KDE, and after enabling the kubuntu-updates repository on both, I can't tell too much of a difference (other than this isn't broken due to stupidity).
You will not have Mint 16 KDE in April (and possibly not even Mint 16 Cinnamon), but you could always install KDE on the main edition. Mint 15 KDE hasn't been around that long, and with backports enabled, you shouldn't miss too much. LTS releases are generally more stingy with the improvements too. Unless you really want to see that Mint 16 wallpaper, I wouldn't bother upgrading until your system either reaches EOL or you break it anyway. So wait until Mint 16 KDE is released, back everything up, and get to tinkering.
Mint does not have an in-place or automatic upgrade system. That's one of the bigger differences between Mint and Ubuntu. If you want a quick and relatively painless upgrade in the future, you'll want to make a separate /home partition.
As my grandpa used to say: "If it ain't broke, don't fix it."
P.S. One more thing, If you want to use Steam, I can't seem to get any of my games to work with Mint 15. It should be fixed shortly after the LTS release.
Running on a 4-slot toaster @ 60Hz