12 L is very small...

i use a 20L bag and it's full with lunch for 1 day and 2 peoples' worth of summer clothing (+ wallets/phones/keys)

i would imagine that one pair of trainers/sneakers would take more than 50% of the bag

as far as Deuter goes, I think of them like a German JanSport, great if you're in college, but the durability is only OK. For similar money, I'd look at Osprey (high quality but too shiny/flashy for me), Fjällräven (very high quality and swedishly subdued) and Arc'teryx (very, very durable and afforably priced with a lot of less flashy options.)

good luck