Welcome to the boards luvmydogiz!!!
I second everything's that's been said above.
Since you'll be travelling on your own backpacking and staying in hostels some, I'd recommend you go with a good size travel pack. Check out your local sports goods stores or STA Travel store. A lot of them come with a zipper-able daypack that you can attach when you're moving from city to city, but disattach for day to day sightseeing.
I don't recommend the regular, school notebook sized backpack plus the duffel bag combination, b/c a lot of the W. European city infrastructures can be more outdated with no easily accessible escalators and/or elevators, plus if you're doing backpacking style traveling, then you'll be walking a lot, so it will become a hassle carrying the duffel bag around.
With the travelpack and daypack attached in one, you can keep it close to your body wherever you go, and it will be more manageable and make you more mobile.
Keep the questions coming!