I'd be really curious what the split is among potential D3x customers if they were asked their preference between the large body and the small one.
Agreed. I think everyone would want small. For getting low to the sand and for travel. If everything else was equal. Hopefully Nikon will do something about their AF sensor coverage on FX before we see the next cameras.
Vertical grips are not much use UW! Extra battery life isn't either. Everything lasts a day - giving you plenty of time to recharge while you sleep. High frame rates, driven by the batteries in vertical grips are also rarely a key UW feature.
Starting from scratch I would take a D300 over a D2X (despite the D300 not having the AF mode I like) and I would take a D700 over a D3 (although plan to get either) and to get really vapourware a D700x (which won't happen for a while) over a D3X. I took the F100 over the F5 (although I have always regretted not owning an F5).
Of course, this assumes that they were announced and available at the same time. Annoyingly, Nikon get the big bodies out first, which means that you have usually jumped before the small body arrives.
Alex