You can cancel a Slicehost server at any time, but you pay for a day if you only use an hour, and the refund is credited to your account as far as I know -- if you need 100 servers for an hour, even 256 slices, its going to cost you $2000, with $1934 credited to your account afterwards, compared to $10 on EC2. If you have money to splash around (your not really spending $2000 for 100 server hours, but you still need it), you'd proberly be better with Slicehost (or some other VPS host), but if you don't, EC2 is a very good option for scaling on the cheap.