I've used nspluginwrapper with very little trouble. The only problem I had was the parent browser window defocusing when my focus was on the Flash element (which might even be a problem with 32-bit Flash). That's a very minor and simply cosmetic problem. Of course a true 64-bit Flash would be better, mostly for convenience and speed reasons, but as it stands there is no such thing. There's Gnash, but I think it only supports up to Flash 7 or so and I've never had much luck with it (it crashed my Firefox).
It's harder to get the 32-bit Firefox to work the way you want. I've had problems ranging from it starting with a different skin, to conflicting with my other 64-bit Firefox, and also broken fonts. Those are just a few hurdles of getting a 32-bit GTK program to run properly on your 64-bit system. You need to install a lot of 32-bit lib packages on your system to get it to work, as well. Everything has to be right, but if you find the right guide you can make it work. It's harder than nspluginwrapper, and not to mention less convenient, at least from my experience. Ever since, I have just used 32-bit Linux since 64-bit offers me no benefits.
I should mention that using 32-bit libs on a 64-bit system to run Firefox32 is different than running it from a chroot, but in my experience I have had even more troubles getting the chroot to work the way I wanted.