We only go with carry on. Seven days or seven weeks. But I realize that this does not work for everyone. I have a 20" Eagle Creek, spouse has a 20" Rick Steves. Sometimes DW will put a few items in my bag as I usually have a bit of extra room. I used to carry a small backpack for book and ipad but I ditched this. Spouse has a big purse. We keep a nylon shell of a bag in our suitcase for the beach/daybag or in case we are concerned about being overweight. More than once I have put things in and carried over my shoulder if I thought the airline was really going to weigh AND I thought that one of us (guess who) had an overweight bag. It can handle 1.5-2Kg in a pinch. When we got serious about carry on only we started with bag instead of what we thought we wanted to pack. That made us prioritize (and focus).
I think that you should consider how you travel. If you are doing trains, rental cars, ferry, etc. you will be lifting suitcases-especially on trains. We also rent a small car so we either need a trunk or more often we get a liftback. Our bags fit comfortably and are not visible to the outside when we rent a Fiat Panda!
Hotels? We stay at all sorts. B&B's, apts, higher end. Will you be staying somewhere where you may have to carry your bags up two or three flights of stairs?
Will you be going from airport to hotel or might you be carrying you bags over uneven pavement or cobblestones. If it is the latter, forget the 4 wheel jobs and stick with a roller with 2 very good inline wheels.
If you are using discount airlines in Europe be aware that the size restrictions are slightly smaller for carry on (20") and the weight restriction is typically in the 7 or 8 KG (16-18LBS).