Commercial compilers offer alot of visual project managment tools that make it easy to keep track of and edit all your source files . I strongly recommend buying a commercial compiler if you plan on writing anything but the simplest mods. In my opinion, it will make things easier on you. Either way, which ever one you choose will have it's good points and bad points, and it will produce Win32 DLL's. To tell the truth, the biggest difference I have been able to find between commercial and freeware compilers is the user interface.
Commercial compilers are for the most part alot friendlier.
Freeware compilers are pretty cool - people write them, keep them updated, and make them available to the public for free. It's a refreshing attitude as opposed to today's software environment. Freeware compilers are harder to setup and use than the commercial compilers, because they are generally written for non-beginners. As far as functionality goes, guess what - freeware compilers end up generating a Win32 DLL when you tell them to, just like the commercial ones. Basically, if you need to get a freeware compiler, go for it.