I am an installer, and your questions are a little complicated and depends on your needs. I hope I can help, and anyone else can chime in..
if you enable PTAT, it record everything on CBS/NBC/ABC/FOX from basically 7-10pm. These recordings do not use up your useable disk space and only use 1 tuner due to some new technology. But this also leaves only 2 tuners to the rest of the house during this period. Dish proclaims you can recored 6 channels during primetime, but you can also only watch 2 live programs during primetime.

If you leave PTAT turned off, you have 3 live tvs during primetime. (4th TV still get's "last one in/watch what the wife/husband/kids are or, watch a recorded show).
One thing, PTAT is not VOD. These shows are recorded and stored on HD every night. They just are on part of the hard drive that does not change your 250 HD hours and will be erased in 8 days. You do have an option of moving a recorded program from PTAT to your saved area, but you must do it manually.
This system has a lot to offer, but it really depends on you household.