If you're going to use different size tyres, I'd also think it would make more sense to put the wider one on the rear. The biggest benefit with wider tyres is comfort, and that's best under where the weight is.
Just to resurrect an old thread; I've just taken delivery of a couple of special offer original Marathons in 700 x 38 size with the intention of putting them away as future spares - as I always like to buy stuff when it's cheap, not when I need it urgently and have no choice but pay full whack!.
However, it has got me thinking about experimenting with mixed sizes on my Raleigh Pioneer hybrid. Currently I've got 700 x 35 Delta Cruiser+ front and rear, with pressures roughly as per the Frank Berto pressure chart for a 60/40 weight distribution. It's reasonably comfortable, considering the terrible condition of the roads, but I was wondering if it might be worth me experimenting with a 38mm Marathon at the rear only and dropping the pressure a bit for a better ride? There is no issue with clearance, because some later Pioneers were factory-fitted with 700 x 38 from new. I've also found by experience that Delta Cruiser+ & Marathon come up about 1-2mm under their nominal size anyway even when inflated close to max pressure.
So the question is, am I just buggering around swapping tyres for little tangible benefit, or am I going to really notice the difference? I've done quite a few miles on the bike so I'm familiar with what is "normal" comfort on 35's. There's also the question of wear rate; Marathons are generally rated as a very hard-wearing tyre, more so than DC+ (which in fairness are also cheaper to buy). With all my previous bikes I've always got through tyres at the rate of two rear ones to every front, so if I fit a very hard-wearing Marathon to the rear, and leave the less hard-wearing DC+ on the front, maybe they'll both wear out at the same time!