I did 10+ days in Germany, Austria, and Switzerland with one carry-on bag (the full allowed size) and a small personal item. I always recommend packing absolute essentials, including any prescription medication, in a smaller bag in case you're forced to check the main bag even though it's within the size regulations.

I used an ebags eTech Weekender Convertible; the Motherlode TLS convertible is an improved version. "Convertible" means that it can be carried as a backpack, although it's not for backpacking. This can be convenient if you have to carry it a long distance. There are lots of functionally similar bags, including some from Rick Steves, LL Bean, and quite a few others.

For the personal item in that trip, I used just an Eagle Pro guide bag, to hold medicines and a few other essentials, a kindle, my phone, and so forth. In the winter your coat pockets might suffice. If you check the main bag (I didn't) the personal item ought to be a bit larger, to hold toiletries and a change of clothes in case the checked bag doesn't arrive with you.

I carried a blazer over my arm on the air-travel days. For train travel I could fit the blazer into the Weekender, but it made it slightly too thick to fit in the sizer, if an airline had checked.

Features other than weight, easy of carrying, and durability don't make that much difference, I'd say. I don't particularly like lots of outside pockets, but it's convenient to have at least one so that you can stash things like rain gear conveniently.

A true backpack, if you like to use one, is an alternative, especially if you are sure that you'll never check it.

The Onebag website has a lot of helpful information.