Well, first to answer your questions- the geometry feels perfect to me, but I do have the '02 RFX which was 1 degree steeper up front and has a higher BB. But at 170mm travel with no ETA, the bike handles great on all but the steepest climbs. On the downhills, it's awesome! PS, for reference, the 170mm 66's are only 12mm taller than the 150mm Z1's.
I pretty much only use my RFX for free-ride, but we most always climb a good bit for our FR descents.I don't really miss the ETA honestly- it's the other parts (heavy DH tires, flat pedals, and 7lbs of pads) that hamper my enthusiasm on climbs. :madman:
OK, with that out of the way: bigger bikes are really fun! Sooner or later the extra travel has us all trying bigger stuff and then you want the geometry/fit and parts to back the frame up. My RFX started out as a trail bike and gradually gained weight as I became more confident. Started at 33lbs and it's easily 38 now. The cockpit has shrunk an inch. No more SPD's. That seems to be the standard experience, most all my friends have gone the same route. So, if I was you, I'd just build the RFX for maximum skill & confidence building, not to mention fun!
Plan to keep the spot, no reason to sour the XC experience with a porky bike. If they are built similar then you might have a hard time choosing between them for those in-between rides. Go for contrast, then you can say, "what kind of ride do I want to do today- XC or FR?"
Hope that helps- just my humble rambling opinions....