I’ve used both a wheelie suitcase and backpack in 2 separate trips.
I would recommend a backpack (80 litre max, which can get quite heavy at 20kgs) with wheels. That way you have the option to use it as a backpack or wheelie as in most European countries there are cobble stone roads, which wreak havoc on wheelie bags. And some places you may have to endure some stairs or lifts (hotels - stairs and quite often small lifts), stairs in some places.
I bought the Deuter 80lt on sale for around $300. Good quality and durable. There are cheaper options, a lot of outdoor shops have sales throughout the year.
If there is snow and you have to walk from the bus to hotel (which happens a fair bit), a wheelie may not be ideal in the snow (winter Europe).
It also depends on how clothing you bring for winter. There are some warm options which are lighter and more compact than others. If you haven’t bought all of your clothes yet. Merino base layers are ideal. Your local outdoor shop is probably a good place for helpful advice. As I’m looking to travel to Siberia / Iceland during winter next year or the year after during winter.
Hope this helps