My wife and I are planning a 4 week trip to France, Spain, and Italy between the first week of October and the first week of November.  I’m look at two different bags right now, one about 3200 cubic inches (about 50 liters) and the other about 3600 cubic inches (about 55 liters).  I am really torn about what is essential.  I’m getting a bag that I can also use on legitimate weekend backpacking trips (following the trip) and take to Europe, so both of the sizes work. 

Does anyone have any thoughts on an ideal bag?  I’m probably going to take a messenger bag or a lumbar pack as a day pack (something I can stuff in the top of my packpack if I need to), but am looking to get a backpack that is comfortable, but also large enough to handle a few bulkier clothing items (to cope with the fall weather). 

Does that sound like the right size?  Any features in a bag that you would consider absolutely essential?  Is a streamlined top-loading bag better (like an alpine bag) or one more designed like a rucksack with lots of side pockets?

Thanks for the advice.