Our department which currently runs a 1000+ core machine and 2500+ core machine for MD simulations is still waiting on jumping onto the Amazon/Cloud bandwagon mostly because it's still not cheaper than owning a cluster for a few years. Granted, being at a large research university means they're not the only clusters on campus and there's an infrastructure already in place to maintain it.
In terms of speed, I would assume Amazon to be faster than most clusters since they probably offer the latest and greatest computers. Lastly, MD is an embarassingly parallel problem (don't ask me how it is) so latency isn't a major issue.