Atmega128 or MSP430 for low power, decent performance?

We need to know your typical application, the latest AVR's are about the same, power wise, in the active mode as the MSP430, the MSP430 are better in sleep modes, the MSP430 has a better A/D, but consumes much more power the the AVR A/D when enabled. Most MSP430 instructions take

I would say pick which one has the best mix of peripherals that you need.

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Most AVR instructions will take 1 clock cycle and the latest Picopower AVR devices are not lagging behind the MSP430 in sleep modes, I think it is slightly better in most cases.

This is mainly due to things like sleeping Brownout detector and the new 32 kHz osc.

If you need self-programming, then it may be of interest to know that AVRs can use selfprogramming over the complete voltage range, while most if not all MSP430 cannot do selfprogramming at 1.8V so you will be forced to run the MSP at higher voltage than the AVR if selfprogramming is needed.

Of course there are no Picopower versions of the mega128 yet, but if the ATmega644P is OK, then you can be in business by the end of the year. Part is sampling and should enter production soon.

Anyone that wants to test the performance of the AVR can download the AVR Studio and run code in the simulator. The IAR C compiler is free for up to 4 kB of code.

with the unrealistic assumption all the dataspace you need can be accomodated by the 32 registers, yes (and the same can be said of the MSP430) but otherwise 2 clock cycles is much more realistic

If you don't need the absolute minimal sleep power and are more concerned with active power and performance per watt, you might want to check out the various ARM devices. Both Philips and Atmel offer very low power chips. We have been using ATmega128 chips in our front panels. In a recent product we decided to go with the AT91SAM7S64 because it is cheaper, lower power and smaller. Although the SAM7S64 uses more power at full speed, the Watts/MIPS is better.

The sleep current is only a significant factor if the unit will spend the vast majority of its time in the sleep mode, for example if it wakes up once a day to do a few seconds or minutes of processing. If it is active 1% of the time or more the sleep current just won't be a factor. Even so, the SAM7 parts are pretty low sleep current unless you use the JTAG port which messes up a pin to drive the quiescent current way up.