Substream 1 is usually the lower resolutions and Substream 2 are higher resolutions.
I use Sub1 at D1 resolution and most of mine are 256 bitrate. Some are a little higher. At D1 you can go pretty high and still keep CPU low. It really comes down to at what point is it too low res for you. If you have a lot of cameras, you can usually get by with a lower bitrate and res than if you only have 4 cams.
It is the folks that insist on Sub2 at 1080 resolution and 20,000 bitrate that do not see the savings.