if i had to choose, i'd go with linode just because i like the founders so much. this is a great step for them to be taking.
So much useful information spreaded everywhere.
I suppose it's a good thing, but I also wish everybody would use StackOverflow and ServerFault ;-)
I think the two sites are complementary to linode/slicehost documentation. StackOverflow and ServerFault are not a general guide on how to setup a server from the ground up. I would only go there if I had a particular problem with my distribution or programming question.
I know, but I also believe it would be great if those two sites served as a hub for companies like SliceHost and Linode to put their support material, which could then be readily improved by the community.