I've done a lot of solo traveling with just a backpack and some with a roller.
I generally prefer the backpack if I'm going to be moving around a lot on my own and taking a lot of public transportation. I like having my hands free for consulting a map to find where I'm going, and I like knowing where my bag is at all times. If I stop to buy a train or bus ticket, I don't have to worry about keeping my bag in sight between my legs at a kiosk. (I have luggage locks on my backpack so no one can pull stuff out of it in these situations.) I can also walk around town with my hands in my pockets to protect whatever I might be carrying there.
I like to use the roller if I'm mainly going in and out of airports, hotels, taxis, and rental cars, or if I'm on a tour and don't have to take care of the logistics on my own. Hoisting the pack onto my back is a needless aggravation in these situations, and there's never enough room in the airplane aisles to get the backpack onto my back without bumping other passengers. The advantage of being hands-free is not as important in these situations.
If I were doing the trip you're doing, I'd use my backpack. (I have the RS convertible carry-on.)
And by the way, I've done most of my traveling since turning 50, and i'm not in the greatest shape, and I've never had a problem walking around with the 25-lb. backpack.