I have the Simple Touch and also a 7" Samsung tablet. I MUCH prefer reading on the Nook Simple Touch, for reasons of eyestrain, as many have mentioned, but other reasons too.
The Simple Touch is so much lighter. You'd be amazed at how heavy the Nook Color or Tablet will seem after you've held it up for awhile. My Samsung is lighter than the color Nooks, and still seems very heavy.
Also, battery life is much longer on the Simple Touch. It's so nice to be able to read for weeks and not have to worry about the battery.
And reading in sunlight is far better on the Touch.
If I could only have one, it would be no contest. I wouldn't give up my Simple Touch for anything.