I haven't had the opportunity to make it to Europe yet but from my experiences trekking through the Americas and Asia I can at least comment on the luggage situation.
If you plan on really never leaving the cities and the tourist traps rolling luggage is fine for you but it's really not my style. I love to get off the beaten path and explore so for me it's backpacks all the way. Be sure to try on several different packs of different sizes, styles and brands - with 20 pounds or more of weight in them. MEC is a good place to start, try on several there and make sure you get the right fit for your frame (most larger packs have several frame/hipbelt sizes available). Don't be afraid to purchase the "cheap" MEC or REI branded backpacks, as they're some of the best valued products on the market, durable and well-thought out. I personally don't find the suspension on their larger packs to be all that comfortable (your mileage may vary - several of my friends adore them), and swear by Arc'teryx and Osprey backpacks.
I do try to get out in the wild wherever I travel and the types of packs that I own reflect that. Almost all of my backpacks have alpine specific features, tons of attachments outside for poles, ice axes, etc except for my Arc'teryx Altra 65. I love that pack and if it fits you well and you have the cash for it/plan on using it for years to come, I highly recommend it. No unnecessary addons, full frontal U-zipper so you aren't stuck with just a top loading option, highly adjustable suspension and fit. No removable adjustable lumbar pack but it's a real winner in my books.
Even in sketchy urban environments I don't go to the degree of locking my bag's accessable zippers or anything like that when I'm travelling around or sightseeing, as some people would. I do however put a
small double gated S-biner through the zippers to dissuade a potential pickpocket or even just to help prevent an accidental opening, dumping your possessions everywhere. Obviously also useful to clip a small whatever to your pack.. I also always make sure that I have a lightweight pack cover, as I've been caught out in rain many times and some of the best experiences I've ever had have been when many other tourists stay in and hide from the weather.
TLDR: comfort > all when selecting a pack, have fun in Europe!