Jackes, to answer your question - I use 12l Salomon and whether it's a short or long run, it works. Does not bounce and you hardly know it's there. Mate of mine runs with the Aonije vests and swears by them - he's even ditched his Salomon pack in favour of it. So if it was me, I'd go for the 10l pack, given that it's the same version as the one my mate uses. I mean we had a quick run the other day of under 2h and he used his 10l pack, I used my 12l pack, both with just one water bottle, cell phone and keys - and we dropped our lights in there as well when it became light enough. But it's not bulky, so if you want one pack, go for the large one. Just... go for a vest, not a "backpack" like the camelbak's from mtb-style.
