If eBay is the place you plan on getting the box from, the prices for both boxes run neck and neck, especially if the TRC-931 is in very good/great condition... I won two 1st edition TRC-931's from eBay for under $200.00 each a little over a year ago... 2nd edition versions sell for more money but are less rare...
Other than that, perhaps the type of music you listen to should determine what box you should go after... If you listen to a lot of Hip-Hop with deep 808 bass kicks, there's absolutely NO WAY I'd recommend buying an M70! For other music with no "BOOOOOOM BOOOOOM", go for the M70.