I don't have enough space for my bottom wisdom teeth. Do I pull them out, or the 2 back teeth (each have 1 big cavity) and let the wisdom teeth grow?

Depends. The answer depends on the extent of the decay in the two teeth and what may be necessary to treat the two teeth, the position of the two wisdom teeth, as well as your age. If the wisdom teeth are impacted, they may not erupt into the mouth, or erupt into a functional position, and could cause more problems. Your dentist and/or surgeon can advise you.
Depends. If the decayed molars are hopeless or have a very poor prognosis then the idea to extract them and let the wisdom teeth grow in is a viable option. If the decayed molars just need a filling, even a large filling, i would suggest you consider having the wisdom teeth removed. Talk to your dentist to see which option is right for you.
It Depends. Once it was common to remove back molars to let the wisdom teeth come but because x-ray techniques didn't give a good picture then that often didn't work - i'm proof! the issue is that the wisdom teeth may not move to fill the space without having braces to close the space. If the back teeth are able to be saved that is usually best. Your dentist will be your best guide on what you need to do.
Dentist's advice. That's a question that your own dentist should help you make. In general it's better to save your other teeth and remove your wisdom teeth. The wisdom teeth often do not just fall into the right spot. If your other teeth are hopeless, that's a different story.