I strap my backpack to my rear rack. If you are carrying a lot of weight its probably best to have your bike handle the load. I have panniers but I don't feel confident leaving them on my bike when I lock it up.

Well designed backpacks distribute the load better. Also some are better than others in regards to ventilation.

You have multiple options for bridging the gap between the backpack and panniers. You have some panniers that will double as backpacks or messenger bags. You can also rig something together to attach your backpack to your rack instead of just strapping it on.