Osprey Xenon 70 or 85 litre

Hi,

I was wondering if I could get some advice re. Osprey Xenon 70 litre and 85 litre.

I'm going for a 3 week trek in south africa as assistant leader (winter season) this July/August and the kit list says "75-80 litre rucksack". DO I go for 70litre Osprey Xenon or 85 litre.. Bearing in mind I want it for normal expeds and I tend to pack more ...but I may need to carry more as Ass Leader and the kit list says 75-80???!!!! Which would be best?

Really confused, which should I choose?

I would get your gear together first. You don't want to buy a huge pack that might never get used again?