My primary motivation for the upgrade from 722 to Hopper/Joey was HD on the second TV. In retrospect, the following are equally important to me:
1. Extra tuner eliminates conflicts.
2. Tuners are not linked to TV1/TV2, so recording one program does not prohibit either TV from watching live.
3. PTAT eliminates conflicts and saves "my" recording space. Your mileage will vary here. Depends on how much Prime Time you watch on ABC,CBS,NBC and FOX.
If you are happy with the 722, stick with it until you are getting the second HDTV. If the additional features are compelling, go ahead and get it now. Just keep in mind that if your SDTV does not have composite inputs, you will need to purchase an RF modulator separately.
Also remote range is better, user interface is a lot better, sling doesn't take over TV2, more recording space, DLNA, more energy efficient, faster, good weather app, and better dual live buffer management. Sure there are bugs, but they're minor.