I've got both the 750 and 900 parked out back. Both are great bikes. At this point, I would say to go with the 900. I think that there are overall less problems with the bike. To be honest, the only thing with the 900 that I am aware of being an issue is same as the 750. Supposedly the stators have a habit of giving out. However, the stators on the 900 can be changed without removing the engine from the frame, so that's a bit plus.
Also, I'm not sure why anyone would say the bike is too slow for sustained highway speed, I can pull an easy indicated 85mph all day long on the thing. A lot of people complain about heavy vibrations at highway speeds, but I don't see the problem.
Options for reducing the vibrations at high speeds would be to swap out the belt gears, and maybe swapping out the rear tire for a bigger one.
The idea for getting the 750 now and then saving up more money for the 900 later seems like a good idea to me.
......I'd rather wake up in the middle of nowhere then in any city on earth.
*2014 VN900 Custom (16,200 miles and counting)