having an inner on the sleeping area makes it extra cosy, and cuts down condensation, but as long as you have a bedroom inner you're not that likely to notice that much difference.
but they're totally different kinds of tent - the nevada (from a quick glance) is a bendy-pole tunnel type tent and is (relatively) light as a feather, while the aspen is a frame tent with a massive pack size/weight (at least my old aspen 500 is, i can't imagine the newer 700 would be any smaller/more compact).
personally i love a good sturdy solid-pole tent, but the choice is personal preference in the end i reckon. check the aspen pack size carefully - we have to put the seats down in the (estate) car to fit ours in (they may have changed this in the newer versions cos it's a bit of a design flaw) and so we just don't use it any more, which is a shame cos it's a flippin lovely tent otherwise.
I suppose the double skin stops the chance of you brushing yourself or belongings against the condensation inside the tent in the morning..I thought only those crappy Sprayway tents had inner tents throughout the living area too (but what do I know really )
BUT Aspen takes longer to pitch - inner first I'm presuming, which is a pain in the rain, also the bigger/heavier pack size is a big consideration as already mentioned.
I've been wondering about this too as we are thinking of changing our Vango, which is a lovely tent but takes too long to put up. The one we are thinking of getting ticks all our boxes except I'm wondering if we will regret losing the double layer, which does make the tent very dry and cosy. Even when it's windy you can't really tell from inside. Still, I suppose we can't have it both ways, I always complain about having an inner-first tent if we have to put it up in the rain!