I have both Slicehost and Rackspace Cloud servers. I'm hoping they bring in more features from the Slicehost control panel, and the migrations will be nothing more than moving images around on their backend (Yeah, right). One big thing missing in Rackspace Cloud servers is the ability to separate billing from technical accounts, and being able to delegate technical control to a different account. This was one of the best features for Slicehost, allowing independent contractors to push out a site, but have billing done through the site owners. Domain names isn't an issue for me (I use DNSMadeEasy) but I can see how it would be a bigger deal later.

Lesser features -- Slicehost has several kernel versions available. Rackspace Cloud does not. Slicehost control panel's UX is just much better.

In spite of all this, I've been moving as many of my customer's servers over to Rackspace for a while now. Most of them don't need the extra bandwidth you get for the slice, making Rackspace Cloud much cheaper when you add in backup images.

IPv6 is a big deal though. Everyone's going to have to touch their long-running servers sometime before the end of this year. With IPv6 Day closing in this June and the eventual migration of residential broadband to IPv6, this is one of those inevitable things that isn't going to magically go away.