Biggest difference between the two is the ability to catalyze the paints being sprayed through a gun. This enables the finish to produce a harder and more durable surface that will stand up to more physical abuse, chemical abuse, UV, high pressure wash, etc. Spray cans typically can't be catalyzed because the paint would set up and become unsprayable because it hardens. Although I think Eastwood does offer a catalyzing rattle can now days. But I doubt it can be used more than once.

Yes, many modern rattle can finishes are decent, but simply won't match the longevity of a two part finish. Never mind the ability of a gun to provide better coverage, better atomization, and more control over the application process.

If you want to do it once and plan on keeping the car for a long time, and don't mind the associated clean up, use 2 part paints. If you're preparing for a quick sale and don't care what it looks like when its out of sight or like to redo parts every few years, spray bomb it.