There is definitely a different target customer and that matters. Rackspace customers are more likely to be classically architected n-tier web sites/apps and AWS customers are more likely to be fully "cloud architected".
The result of this difference is that a Rackspace cloud physical host is not going to be as heavily taxed resource-wise (at least on average). Add to that the cpu burst by default versus cpu cap by default and cpu is definitely the one area where differences will be widest.
One thing I will say though, and many may disagree with this, but I have yet to see a benchmarking study that I'd trust. Real world operating performance of a server is just a very complex thing and any attempt to distill it down to single parameter comparison is going to compromise some element of that complexity. I can show you comparison studies that show any of the cloud providers to be better than the others, including AWS, Rackspace, Linode, Joyent, etc. So take these things with a grain of salt.
> ny attempt to distill it down to single parameter comparison is going to compromise some element of that complexity
This is extremely true not just of this but of comparison in general. Turning something complicated into a natural number below 100 and then using > and < is pretty limited.
Doesn't seem likely. VMs impose limits on resource consumption (though Rackspace allows CPU bursting, from what I understand). It's not like the old days where you could get lucky on a sparsely-populated shared server.
Rackspace cloud servers are basically what slicehost offers. They don't have cpu bursting. Instead each slice on the host is guaranteed a minimum amount of cpu and any unused capacity is available for other slices to use.
So if you have four slices on a host and 3 slices only use 10% each of total cpu capacity. Each slice is guaranteed 25% but they aren't using all of their guaranteed cpu. Your slice can then consume the remaining 70% of the cpu on the host. (Minus virtualization overhead)
Unlike bursting, you don't get throttled until other slices need cpu. Your slice can happy keep consuming 70% until the other slices start asking for their share.
I know Cloud Servers originally started as a direct copy of slicehost with hourly rates and end of the month billing instead of monthly prepaid billing (prorated per day). A few things have changed since then and I don't know if slicehost and rackspace are still offering the same system any more.
I do know that the vm environments are the same. I'm just not sure about some of the other bits.
If you go by the look & feel of their web interface then it certainly still feels like a bolt-on. I also remember various of their server ips to have PTRs to slicehost names, but that may have changed since I looked (a couple months ago).
