Python or IronPython

There are a number of important differences:

1. Interoperability with other .NET languages. You can use other .NET libraries from an IronPython application, or use IronPython from a C# application, for example. This interoperability is increasing, with a movement toward greater support for dynamic types in .NET 4.0. For a lot of detail on this, see these two presentations at PDC 2008.
2. Better concurrency/multi-core support, due to lack of a GIL. (Note that the GIL doesn't inhibit threading on a single-core machine---it only limits performance on multi-core machines.)
3. Limited ability to consume Python C extensions. The Ironclad project is making significant strides toward improving this---they've nearly gotten Numpy working!
4. Less cross-platform support; basically, you've got the CLR and Mono. Mono is impressive, though, and runs on many platforms---and they've got an implementation of Silverlight, called Moonlight.
5. Reports of improved performance, although I have not looked into this carefully.
6. Feature lag: since CPython is the reference Python implementation, it has the "latest and greatest" Python features, whereas IronPython necessarily lags behind. Many people do not find this to be a problem.

There are some subtle differences in how you write your code, but the biggest difference is in the libraries you have available.

With IronPython, you have all the .Net libraries available, but at the expense of some of the "normal" python libraries that haven't been ported to the .Net VM I think.

Basically, you should expect the syntax and the idioms to be the same, but a script written for IronPython wont run if you try giving it to the "regular" Python interpreter. The other way around is probably more likely, but there too you will find differences I think.

Well, it's generally faster.

Can't use modules, and only has a subset of the library.

Here's a list of differences.