IMGBurn was made by the same guy! And yes, IMGBurn will only burn an ISO file! I use it because it is incorperated in DVDRebuilder.... Rebuilder can be set to make an ISO file and then automatically burn with IMGBurn or DVDDEcrypter! They just updated IMGBurn less than a week ago and it's a great little program!
Nero is a different animal..... I just upgraded to Nero 7!!! it has it all and I am still learing it even though I know most of it! So, to answer your question..... IMGBurn does one thing while Nero does soooooooo many things, it's more complete. But, for what it does I use IMGBurn, too!
If yo talk just about burning an ISO, you could argue until the cows come home about which is better, IMGBURN or Nero.
As stated above, Nero can do so many other things but tries to put a simple user intreface on the front end. Unfortunateley a program that has so many features will always take much time to get used to.
If you like things broken down into specific tasks, with very specific support forums available then the alternative programs offer there own advantages.
Like most people, I end up playing with all of them, understanding their use in varying degrees of competenace and never quite deciding to ditch one or the other.