First, welcome.
The HR21 and HR20 are identical except that the HR20 has an Off The Air tuner built in, the HR21 does not. When you order, you're actually just ordering the DirecTV+ HD DVR, which means you can get any of the current models. They didn't bring you the wrong model, they just brought you a different model than you were expecting.
The HR20-700 is my most stable machine (of 4). I'd stick with it.
The HR21 and HR20 are identical except that the HR20 has an Off The Air tuner built in, the HR21 does not. When you order, you're actually just ordering the DirecTV+ HD DVR, which means you can get any of the current models. They didn't bring you the wrong model, they just brought you a different model than you were expecting.
The HR20-700 is my most stable machine (of 4). I'd stick with it.