It looks like you've already done a lot of homework, and you're on the right track with a Harmony.

All of the models you mention(even the TC-30) use the same database, are programmed the same way, and work basically the same way.

The 720 is probably the pick of the bunch at the moment...it is basically an 880(rechargeable battery, colour screen) with the form factor(rectangular rather than peanut shaped) of the 520/550. That being said, the 550 will still work fine and do all the same things if you want to save a bit of cash. I think the 520 might have been discontinued???

As you have noticed the 720 has 2 more LCD buttons than the 550(and the 880 has 2 more again), but this doesn't mean you are only limited to that number of activities. All the models can have up to 255 activities...over multiple pages if required.
This would mean in your case you could have "Watch TV bedroom", "Watch TV livingroom" etc.

They all allow one-touch operation as you suggested(maybe 2 or 3 presses if the activity isn't on the first page....depends how many activities you have).
Selecting an activity turns on all devices needed for that activity, selects the correct inputs(even if it involves toggling through inputs), and turns off any device not needed for the activity(you can customize this). When this is done, the remote is setup to control the activity, rather than a particular device. This might mean(for example) the transport buttons control the DVD player while the volume buttons control the receiver.
You can also select a dedicated device mode for each device that will have all functions of the factory remote available.

All programming, firmware upgrades etc are done on-line, so the only proviso is that you need an internet connection to set them up. The upside to this is you have access to a huge database of devices...by far the largest available.