Looks like both are optimized for 12V operation. Sundown is gonna be cheaper. If you're buying direct, Sundown says it's on back order. If you can get a good price on the DD from an authorized dealer, go for it.
Without having tested the Sundown... I have to believe it's going to be a tad stronger. I know the M2b is a bad ash little amp though.
You can't go wrong with either.
Doesn't Sundown have a dyno of the SCV2k out there somewhere?