I think both have great support. They both have active and lively IRC channels where staff hang out. It's usually very quick to resolve issues with slicehost and linode.
The pain point for me at slicehost was bandwidth - they charge a lot, linode charge a little. Also far more options on linode etc etc once I moved that all added up to a better experience.
I just checked my email. In the last year, I've had three incidents with Slicehost, all connected to issues experienced when migrating VPSes. All were resolved in minutes.
Not to knock Linode, or Rackspace Cloud, or EC2, or whatever, but none of them have ever been in the foxhole with me at 3 AM when the server was down and got it back up before customers noticed.