Defiantly go with the one pack. Two packs will add alot of extra effort during your 7 weeks traveling.
When i chose my pack, it wasnt really based on the length of the trip. I packed for one week with nice shirts for going out, long pants for the cold ngihts, shorts, and tshirts. It was a enough to last me for one 3-4 days. Then i wore everything twice. That way after every week i could wash clothes and be ready for the next week. I traveled for 5 weeks. My pack was 35 liters. It was really small compared to other packs. Some people had as much as 70 liter packs.
No matter the size of pack you get, you will always be able to fill it up. The less luggage you have the better off you will be. Thats assuming your bouncing from hostel to trainstation/airport to hostel and so on.
I could have gone 10 weeks with my pack, each week i just washed and i was ready for the next. THat way you dont have to bring weeks worth of clothes. People only saw you one day. So if you wore the same thing the next day or alternated days, no one would never know. Plus, everyone else is doign the same thing haha
Pick a size that would enjoy carrying around everywhere yo ugo and see what you can fit into it. Pick one with a daypack if you can. Like i said earlier, no matter what size you get, you will alwys be able to find somethign else you could take.
i would go to the local outdoors shop and try on different packs. Get a feel for different sizes and then decide about small or big you want your pack to be.