I would have to say from what it seems, people with the first versions of the Mac Pro, like myself are better off running 10.5.8. Since we have the EFI32 it is pointless running 10.6 which I found was very memory hungry. I would see it go through my 6G of memory like it was nothing, & in 10.5.8 I haven't seen it use past 3G. I too went back to 10.5.8 & haven't had a problem since. Perhaps when we upgrade our systems to something with EFI64 then perhaps we would have a better chance with Snow Leopard, otherwise save yourselves some headaches.
That's my 2 cents worth.
In what way are you better off running 10.5.8? Kernel 64? Big deal, it doesn't run by default on any of the workstation hardware including the latest Mac Pro.
That is the only potential difference EFI64 would make. Memory management is down to the OS, not the version of EFI you use.
Now for saying that Snow uses more RAM, I believe you are right there, and I use an EFI64 Mac. I think its memory management could use some more tweaking.