Differences between IIS 5.1 and 6.0

There are litteraly a bunch of changes in IIS 6 compared to IIS 5.1. Let's give an overview:
1. Better security: IIS is not installed by default and after installation of IIS it comes locked down on the machine.
2. Application pools: new application model to load applications in a sandbox under a certain security context which is configurable by the administrator (so you can elevate priviles of a certain app without affecting others). App pools also have a recycling mechanism and replace large parts of machine.config configuration concerning the execution of the web app.
3. Smoother "integration" of ASP.NET (no separate installation, comes right out of the box, but needs to be enabled).
4. Editable metabase in XML.
5. No limits on the number of concurrent connections (which is the case in IIS 5.1).

So, for the hosting infrastructure the decision should be IIS 6 instead of IIS 5 (and of course instead of IIS 5.1 which is not meant to be used in hosting environments at all). However, if you want to run W2K3 on the development desktops, consider to use a Virtual PC to run W2K3 in, under Windows XP (actually I'm not following this recommendation myself and I run W2K3 as the primary OS on my laptop). Be aware about the fact that W2K3 has less support for laptop-related things (such as BlueTooth, Wireless LAN with WPA, etc). Also the advantages on the field of XP SP2 are not in W2K3 yet (will come in W2K3 SP1 which is now in RC1 phase).

For the Difference between IIS 6 and IIS 7. Please refer to the link given below.
I can also write about them by my thoughts, but i won't because he explained very clearly about the main difference, and definitely i can't reach him so please look into it.
URL: http://www.iisworkstation.com/2008/09/iis-6-and-iis-7-how-different-are-they.html