In my experience, rust doesn't necessarily mean the metal is actually bad. I had a few spots on my car that looked terrible, but after some sanding turned out to be just fine.
Personally I'd make a list for each car on what needs to be patched and replaced then compare. It might make the decision pretty easy.
As for what to replace, there are many on here who can elaborate better than I, but generally anywhere rust has eaten through the metal and impacts the structural integrity of that area should be cut out until you hit clean metal.
When reviewing the list of each, it might help to know both pricing for replacement, along with complexity and skill required.
