10.6 switched the architecture from i386 to x86-64. Since many third party applications were still i386 binaries at the time of its release, both versions of every system library had to be loaded. That did increase memory use in some ways, but it's not really a problem anymore unless you use Word 2008.

But that's about it; 10.6 is much faster and more stable in every other way, that being the entire point of the release.

Of course, I should say 'was', since it's not even the current version. Unless you're posting from a time warp.