I find it odd that most who get a Keepall go for the 55. The only use I can imagine for such a big bag would be as luggage, and you can't bring it as a carry-on, not with any airline I've used at least. From looking at the dimensions of the different sizes the 45 seems the most versatile of the lot, in my opinion of course.
really? I went on line and a lot of people said it can be brought on as a carry on.
I ordered a 55. When I looked at the dimensions they were only about 2" bigger. We will see.