> Slicehost can more easily upgrade your plan. Linode has to switch your box should you want to move plans. Often Slicehost can move plans while keeping you on the same box.
I'm on slicehost, and when I upgraded ("resized") a slice, it was not a "live" operation. I don't know whether it's physically the same machine, but the old slice had to be shutdown, file system copied, and a new slice booted.
From the "Resize your slice" page:
Step 3: Current slice is shut down and migrated
You will experience a downtime varying in proportion to fullness of the filesystem.