Hi JB,
Firstly, this is solely my opinion. Whichever decision you will choose should be what's best for your future endeavors.
I understand that you may be talking about the development IDEs in your question, but I will frame my answer toward the languages themselves.
If you are going to create applications with user interfaces you should probably move to .NET.
VB6 (which I still help with development with on our older projects) is simply not close to the robustness of .NET.
Also moving to .NET programming allows you to pick up other .NET "languages" as well. I started off as a junior programmer in VB.NET and was forced within a month of a job to start coding in C#. I thought the world was ending. After playing
around for a couple of weeks with C#, I realized most of the differences were syntactical.
Included below is a link explaining the major differences between the two:
http://visualbasic.about.com/od/imhoinmyhumbleopinion/ss/FiveChanges.htm
Good luck!!!
- stratios
