There are quite a lot of things you can do in with ControLogix that can't be done with MircoLogix. Examples would be function block programming (especially handy for PID loops), user created functions, additional math functions, additional protocol support using backplane mountable modules, and...
Depending on where you live, there are several codes you are required to follow. In the USA, you will be safe with NFPA 70, which is also known as the National Electric Code (NEC). YOu will specifically be looking for the tables unde NEC 310.16 (?) I believe. If you don't feel like ponying up...
I would suggest using one of the several processor based relays that jraef suggested. I have used several of them in the past, and they all provide the necessary protection you need, and installation can usually be accomplished for under $10,000-$12,000 or so depending on your setup and vendor...