Linode offers slightly more in terms of specs, but they only recently switched to Xen virtualization (from User Mode Linux which is much slower). Their service is less mature.

Slicehost is really popular and with good reason. Linode might turn out to be just as good, but I know the wonderful service I have gotten at Slicehost and how many other sites in the web 2.0 community are there - heck, the Xen blog itself is on Slicehost and not any other Xen provider.