I think you're making the right decision, but in case somebody else reads this, remember the larger size will be legal for carry-on on British Airways.

I am currently thinking whether to buy the larger size (it's currently on sale because of the reasons discussed) since I'm using BA a bit at the moment, but really in two minds as I might be flying Lufthansa next year, then it will be too big (but not big enough to be a regular checked item).

As ever, it's price vs. utility and its a tough call.