I will chime in as I have both an untethered and a tethered ATV2. In my opinion, the untethered is the only way to go. Just so nice to not have to worry about it (power outage, disconnect, XBMC freezing forcing a reset). Now, while it is pretty unusual to have to worry about those things, it does get a little annoying to not be able to restart the ATV without hooking up the computer. Having the untethered makes it nice to have the flexibility to take it with you anywhere you go (hotel, traveling, friends/family house) or just to move it to another room.
To me the "hockey puck" sized device is invaluable to have the above flexibility to move it anywhere you like without needing a computer. I have all my ATVs Velcro'd to the back of the TVs, so I can simply grab them and go as necessary (a lot of times I sit outside and have a TV for my bar that I plug one into and watch XBMC outside). I have a few spare power cords for my setup so I can leave all the cords in place and just grab the ATV and spare power cord and go.
The tethered ATV is in our master bedroom and has served us well, but there have been times I have needed to reboot tethered, not a huge deal, but still that is a stationary device, as the untethered ATV is always my go to. I just had to make sure the mounting of the tethered ATV allowed easy access to the micro USB for tethered booting (wall mounted TVs).
That being said, there is zero difference in operation when running, but still the untethered just has that additional flexibility. As for using it for Netflix and HBOGo, I doubt you will see the need to use those anymore after using XBMC. I previously had both Netflix and HBOGo and had cancelled Netflix within a week of using XBMC and have stopped using HBOGo all together, as XBMC will have all the programs those apps offer and way more.
All in all, that is my experience with using the ATVs/XBMC for the past 2 years, hopefully that will help with your decision.