dmips
Mips is architecture dependent, DMIPs supposed to be architecture independent.
Both are utterly useless when you want to determine whether a special device will provide enough performance to execute a given task. to be fair though, MIPS are even a little bit more useless than DMIPs.
Use a small program that represents an important part of your task and execute that one on an evaluation board. That's the way to go.
For ARM or 51 and I am sure for many other devices too, there are simulators that can provide a good approximation.
hth, Bob