http://coronalabs.com/products/corona-sdk/faqs/ has a nice table.
I think the main difference is that Starter does not allow IAP (though you can, I think use Adverts)
Basic and Starter do not have Graphics 2.0 which gives a lot of extra graphic functionality. Pro is for ‘serious amateurs’ and Enterprise for professionals. Pro gives you most of the functionality, but you can’t build in anything other than lua and you have to build via Corona. The first doesn’t matter too much in most cases, and the second one isn’t a huge problem as you develop in the simulator.
I think the basic idea is:
Starter : try out, see if you like it.
Basic : first project, one man band
Pro: serious about it, but either work alone or in a small group
Enterprise: do it for a living.
Everything is a trade off. There are certainly bits of Corona I’m not enamoured of. But it is balancing support, developer activity, company stability. Look at the others, try them out, most of them you can try for nothing.
Look to see how many users it has - this affects both the speed of support (see if questions are answered by non Corona employees*) and the stability of the company.
I asked on one form what happened if the one developer was run over by a bus. This was treated by some as a joke question, but if you are doing it professionally support and stability matter. Ropey APIs can be worked round, usually. Apple especially continually bu**er around with the development environment, which can be a problem.
* this isn’t critical of the Corona staff who provide answers rapidly, but gives you some idea how many people are using it.