I use the SLR body a ton. It's my bread and butter. I've been using the H3DII-39 for years and have been very happy with it. And with a quick mirror lockup, it's as hand-holdable at slow shutter speeds as th alpa SWA. (just a lot bigger and heavier.) Yep, lots of H glass. If I went the Phase back route, I'd likely stick with an H4x or H5x.Other thing to consider is how much you will use the SLR body. Everyone is entitled to their own opinion, but for the times I rented I found the H body excellent (at least for a DMF SLR) and the Phamia body below average. (ergonomics, reliability, responsiveness.)
I guess you may also have a case of H glass if you're upgrading? It's a hard pill to swallow when jumping ship requires you take a hit on glass as well as body and mount.
One thing I will say though, I would find it a little tedious having to power an H4 back with an external battery via the firewire port. I know Silvestri and Quantim make good or ok products, but it seems like a cludge to me. The H5D-50 is well discounted at the moment and, at least where I live, nearly half the price of the H5D-60. I'd go this way if I were buying new.
I agree that powering the back with an external battery is cumbersome, but the ImageBank II has been reliable and quite portable. Obviously the H5D series is preferable, but is it worth paying an extra $5-6k to not need an external battery on a tech camera? I'm a working professional, and I have higher priorities for that 5-6k. Marketing, testing, retirement planning, home repairs, buying the parcel of land cross the street, possibly a kid.