If you're traveling as light as you say, the weight in panniers (or saddlebag) won't affect handling any more than the weight on your back in a backpack, but panniers would relieve you of the discomfort of shoulder straps and a sweaty, weighty back. Conversely, the weight in a backpack raises your center of gravity, which could have a negative impact on your bike handling.

If I was traveling that light, I would use a handlebar bag and/or a racktop trunk bag. I wouldn't even consider a backpack for a tour from Amsterdam to Rome, even for the lightest of loads.