Great question. It's not an either / or thing. Radian Studio is not a GIS but a tool for spatial data engineering that is often used with a GIS, or with a DBMS that contains spatial data. It's also often used with databases that don't have any spatial data in them, since the Radian toolset provides all sorts of slice and dice capabilities useful for general purpose DBMS work in addition to spatial work.
There is useful discussion on that in http://manifold.net/info/radian_faq.shtml
It's natural that people who work with Release 8 will also use Radian, side by side, just the way somebody who works with Arc or MapInfo or Q would use Radian. You can get a great feel for what Radian can do by downloading and using Manifold Viewer, a free product that in the formally released version tracks Radian Studio and which in the Future Viewer series of builds tracks Manifold Future.
As you may have read in the forum, in particular the announcement, in a series of open builds called "Manifold Future," Radian is now evolving from being a spatial data engineering tool into a GIS, the next generation of Manifold GIS products.
At some point in the Manifold Future evolutionary path, some people will say that what was once Radian is no longer just a spatial engineering tool but in fact has become a GIS. Further down that path, more and more people will start saying that and at some point people will start saying it is a totally awesome GIS. You can judge for yourself where all that stands by participating in the process. You'll need a Radian license to use Future, but you can always try Future Viewer to get a taste. I think pretty much all Radian licensees have switched to Future, since Future is already far beyond Radian Studio in terms of feature set, and Future is rock solid and reliable.
How all that compares to what you do with 8 depends on what you do with 8 and at what point you jump into Future. Besides the practical aspect of trying to summarize over a year's worth of commentary in this forum and a few thousand pages of documentation, which is not realistic in this short post, it all really does depend on what you are doing and what you require.
Only you can be the judge of that, so I'd encourage you to get into Future, either using Future Viewer or a Radian license, and start the learning process. The sooner you start with Future the quicker and better you'll be able to launch with the next generation of Manifold GIS. Plus, on the way you'll have huge capabilities to augment what you are doing with 8, for example, vastly more formats and data sources from which you can harvest data.
If you are doing significant work with GIS using any GIS product, it's hard to imagine a situation where getting into Radian / Future won't be a highly worthwhile addition to your toolset.