stellaris cortex-m3 power consumption
You need to be more specific, which particular member of the Stellaris family and which one of the LPC21xx family.
While the core M3 might use less power than the ARM7, there are so many other criteria that have a significant impact on the power consumption. One is the process the devices are manufactured in. Stellaris devices use an older, larger geometry process than the LPC2000 devices, that results in higher power consumption running at the same frequency but an older process provides better standby currents in power down.
Depending what you want to do, an M3 core can provide a better performance, e.g. if you create algorithms with lots of divide instructions, which are not available for the ARM7. In such a case, you can run slower with an M3 performing the same task ask with a faster ARM7.
Most important however is how the connection to the peripherals is done. If many peripherals are clocked all the time, the power consumption will be a lot higher.
In my past, my company did a few measurements using an LPC2103 and a low end Stellaris device. Running the same clock, the LPC came out a lot lower but we did not use the M3 extensions, so the comparison is not apple to apple.
IMHO, Luminary can and will improve the power schemes on the Stellaris family while NXP will introduce more and more LPC1700 devices
http://www.lpc2000.com/m3/ (M3 based).
At that time, your question will become void, today it might be a valid question though.
Bob