I bought several expansion packs when I was using EZ2, they were great additions to the software since Ez2 had very limited mixing abilities.
I've had SD3 for several years now and have never bought an SDX. I'm sure they're great, and I'm probably missing out on some things, sure...
but SD3 comes with a VERY robust preset list that gets me in the ballpark of where I want to go everytime... and then tweak from there to taste... swap drums, change overheads, reverbs, compressors, etc...
I can't seem to find an actual list of all the presets online, but there are well over 100