Generally, the more radiating area you have the better, so in that respect the RM-2 has an advantage. But, the 626Rs tend to be easier to place, and have a much smaller baffle, so will tend to image better. But they will not play as loud as the RM-2 will, so if you listen over 100db regularly, it might not be something you want.
I am willing to bet that if you are using the stock crossover in the RM-2, that the auricap upgrade on the 626R w/the new ribbon tweeter will give you more transparency and better imaging.