The M&S one is actually really nice in the 'flesh'. I bought it at the weekend. It doesn't look the way she is pushing out her tin petite little arse! My bottom is a fair bit larger, and it looks ok in it. TBH I didn't really look at the back of it, but put it on with some 50denier tights and some pumps, and I really like it. of course, thats the best thing with denim, you can wear anything with it!
Also, the length of it is perfect. It's just a bit above the knee, kind of mid thigh on me. Not too short, not too long. Comfy and just right really. Had a fair bit of stretch in it too. Essential for me, getting down to play with DD.
Try it on and see before you scrap te idea, you might be surprised Plus, for the money, I think it's a good buy.