consumer reports is about to test a C300, on their web site they talk about the MB-tex. While not commenting on whether its better than leather or not, they do say that its a very good material.
personally, I've had leather in several of our vehicles, including the our family mini-van and I prefer it. in the end, to me it is much easier to keep clean and maintain over material, and has less cracks or splitting than vinyl. Not having MB-tex, I can't compare it to leather, but based on the responses here and on other forums, you probably won't make a mistake ordering either leather or mb-tex. I suspect that they both will hold up quite well with some of the same basic cleaning.
Of course, with anything, if you only clean the peanut butter & jelly, melted crayons, etc. off of your back car seats once a year, then it doesn't matter what you have, they'll both be ruined. But if you clean up spills as they happen, do a good vacuuming a couple of times a month and wipe down the seats with a damp cloth once or twice a month, then your leather and your mb-tex will probably look the same after 4 or 5 years. The only difference is that you will probably want to apply leather softner or conditioner a couple of times a year. leather is not that hard to keep up with.
Considering how good MB-Tex is, It really doesn't matter what the vehicle has except to you. You might want to save the 1500 and put it towards a panaramic roof so the kids will have something to look at on trips!
Personally, I prefer leather because I figure if I wanted vinyl, (even good vinyl like MBtex) then I would have bought a chevy.