I was thinking the 121 because US airlines seem less strict than international and is is a bit smaller in terms of linear size, so I would get some flexibility.

Anyway, I got the 122 today. Strangely, the 122 spinner model seems at least an inch shorter despite identical advertised dimensions, but I have never had a spinner before and found it a bit awkward to use.

I am probably worrying too much about this because I have been using an ancient Delsey for years that is not even close to the maximum dimensions and would hate to have to start checking.