I have the 50 with strap, and the strap is very useful when you have to carry the bag some distance. It's more comfortable to carry it across your body, that way it won't hit your legs and just rests on your hip. By the way, I'm only 5"0.
I have the 50 without the strap - I just took it on it's first trip and never once regretted getting it without the strap - I dont like how they bend in the middle with the strap and I just prefer handhelds.