I have both.
The 2d is a quality recorder. The option to record 4 channels sets it apart from the M10.
The M10 is still about $210, but the 2d got cleared out around $99. Is the M10 worth the price difference? If you only do short recordings where battery life isn't important, the 2d will work fine. For longer recordings where battery life is more important, the M10 is the clear winner for battery life, for easier menu navigation, for the rotary gain knob, and for being just the right size to fit a shirt pocket, IMO.
With the 2d being discontinued, the current question for some buyers will be whether to choose the M10 or the Tascam DR 60d now that the 4 channel 60d is on sale for about the same price as the M10.