My guess is that the 55 mil stuff is high volume (Sam's, Costco) and so it cost less per unit. The 75 mil costs enough to think permanent and your almost to solid epoxy professionally installed at that point.
We finished up the moving this weekend and so I had an opportunity to abuse the stuff pretty well. Dropping things, tossing boxes, furniture, you name it. 1400 pairs of dirty boots and sneakers. Friggin dog hair everywhere. Even rolled/dragged a refrigerator on it. Bunched up a bit and looked a little worse for the wear, but when I got everything arranged and swept it out it looked fine - no holes and no tears. Car is happy sitting on plastic and sitting under plastic. Owner is happy that the car is happy.
Seriously - this has got to be the best low cost solution out there. It's about $1.30/sqft and if I screw it up badly enough I can get another roll. If I move it comes with. And no I did not buy a load of vinyl futures last week <img src="/forum/images/smilies/968/wink.gif" class="smilie" alt="" />