I would guess Amazon moves things around more, but most VPS providers, which target more the usecase of you running a VPS 24/7/365, put you on a server and leave you there, rather than rebalancing for load (migrating a live guest transparently isn't that easy).
They should still set things up so you get reasonable baseline performance, even in the high-contention case, rather than overselling the resources. But you can end up with quite a bit of performance variance in the upwards "more than your fair share" direction, especially for I/O, if your neighbors are quiet. If you're on a 32-guest machine where everyone else is idling on IRC or doing nothing, you get a whole disk to yourself; if everyone is doing random alternating reads and writes, you get a 1/32 of a thrashing disk's worst-case throughput. Usually you get something in between.