once you pass the 4 gig of ram mark, all of the distros become usable for you, so -- in your shoes -- i would decide what's the most important things you need.

for me, it's a distro that doesn't require to me to upgrade beyond the using package/kernel updates and that's something like debian where i can stick is a release for a really long time w/o having to reinstall when the next version is released.

most distros seem to a have a migration path, but it's rare to find one that doesn't have any hiccups.