Cubase/Nuendo guy. It's mainly due to workflow, but also need N3 for post-prod jobs I get in from certain clients.
I've found Cubase 4 to be buggy as hell on my system so I've moved back to Nuendo 3 until the next patch is out and I have a bit more time for testing. C4 is crash city for me....thanks Steinberg! The only function in C4 that I really miss is the ability to re-order inserts. I still need Nuendo for the post-prod jobs I do but I'll think twice before buying the upgrade when N4 is out based on my experiences with C4.