Gear in hand verses what may come. The 33x44 51mp Sony chip in the 645z is amazing, will do great things, and is available today, even used. Waiting for some mystical camera to show up, and if your lenses will perform as needed is a mixed bag at best.
If you can swing it, go with the Z, either after renting or finding a flip-able used one. It'll either work for you or not. I happen to find it a lot of fun, more so than the high-MP 35mm gear.