The general consensus is that the 612s tend to be a little more buggy than the 622/722/722K and they are usually the last to get updates. Other than that they are well-rounded receivers with plenty of time for DISH to get the crippling bugs worked out. Back in the day the 612 was pretty awful but that was due to buggy software which has sense then been fixed for the most part. At $10 a month (plus the single $6 DVR fee) compared to $17 a month, they are definitely a better value when not using the TV2 (SD) outputs. In your scenario, it would be $6 + $10 in receiver fees, versus $6 + $17 for the 622/722/722K.
The 722K has the ability to record 2 OTA + 2 SAT (612/622/722 are only 1 OTA + 2 SAT.) Whether or not that is worth $7 extra is up to you, plus it needs a $30 OTA module to accomplish such. The thing is, DISH, "considers" the 622/722/722K "equal" therefore you may end up having to play "tech roulette." When the tech calls to inform you that they are on the way ask them if they have 722Ks on the truck. If not, spare them the trouble and tell them to cancel the install. You will need to set up another appointment and hope they have 722Ks on the truck next time.
EDIT: Oh, forgot, 622/722/722K support PiP, 612 does not. Again, is that worth $7 extra or not, up to you.