Had a Peavey fire bass with a 4x10 and a single 15 cabs. I stuck the 4x10 on top of the 15 as you do. It was quite a while, a couple of years actually, after I started using it that I noticed that the 4x10 was a lot bassier than the 1x15, infact, it was only after the 15 blew and I replaced the basket, a Black widow, that I noticed the difference while checking the cabs separately.
So which cab to put on top? I'd say don't use different size speakers but that's just my opinion.