FWIW, I use both P6 and P7.  I would really really rather use just one but...

P7 renders much faster and with more stability, though P7 will sometimes crash suddenly during non-rendering activities (more so than P6 from my experience)... and

P6 has a cloth room which actually works for multiple cloth sims whereas P7 cloth room is a disaster, especially for re-running multiple sims.

I do lots of animation so I need both functional cloth sims and fast rendering.  Currently, then, I use P6 for all steps up to rendering then I turn the PZ3 over to P7.

Do note that if you save a figure or pose to your P6 runtime while using P7, then try to load the new item while in P6 it will give you an annoying (and apparently worthless) popup stating that the item being accessed is from a newer version but Poser will deign to try to open it for you anyway.  I've never had it fail to load P7 content in P6, but the popup annoys me enough that I usually try to manage my runtime content only through P6.

Good luck, and I think we can all hope that Poser 8 (in addition to being a great quasi-verb for using Poser... i.e., to "poserate") lets us finally get one version which does everything better than every prior version.