I sometimes have a harder time with my right foot than my left foot aswell. I think it's because by the time i've leant the bike a little bit to the left (in anticipation of resting my left foot on the ground), my ankle twisty motion for my right foot isn't quite at the right angle - due to the angle of the bike (did that make any sense?). Anyways, I've got the tension of my right pedal backed off a bit more now, so it's a lot easier to come undone. Only problem with having uneven tension on the two pedals is that when i bunnyhop, my right foot sometimes comes right off the pedal!

Also, not too sure whether it's possible with the type of SPDs you're using, but you could try angling the cleat in your shoe a little bit towards the direction that you go to unclip - that way, it takes less of a turny ankle movement to unclip.