Al-
As you may know, DotNetNuke (DNN) is a spin-off of the original IBuySpy (IBS) portal which was developed for .Net 1.0 and later reissued as the Portal Starter Kit (PSK) on .Net 1.1. To my knowledge there is no further ongoing development on IBS/PSK. Development is continuing on DNN with many additional modules available. DNN is written in VB although you can develop modules in any language. IBS/PSK was available in C# or VB. There is a C# spin-off of IBS called Rainbow Portal.
Personally I think that since DNN has more features it is more difficult to develop with, but IBS/PSK is, for all intents and purposes, dead so it's probably worth the extra effort to learn DNN. Also since more people are using DNN you'll have a much better chance of getting replies in the DNN forum.
My two cents worth,
Mike