Running following scenario with a Buhlmann deco model:
First dive: 100ft, 20 minutes bottom time
30' minutes surface interval
Second dive: 100 ft, 20 minutes bottom time
This is pushing it (to say the least) for recreational diving.
With air, then nitrox 32, I get 26 minutes and 25 minutes runtimes.
With nitrox 32, then air, I get 25 minutes and 29 minutes runtimes.
As you accumulate more deco, it starts to make more difference: if both dives have a bottom time of 25 minutes, air first gives you 36+34 runtime, nitrox first 30+46.
So my conclusion would be: for similar dives, nitrox last, but for recreational diving the difference is negligible.