I haven't faced a similar situation yet (I'm going to have to eventually), but I can tell you what the difference is (hence the price jump).
Professional
Will get you pretty much what Professional gets you now. You can develop everything you need to and do basic debugging (which is what you probably do now).
Premium
Premium adds in a couple features that aren't found in Professional. You get Static Code Analysis, Code Metrics, Profiling, some Testing tools, and read-only diagram support.
The value add here comes if you're working on a medium sized team. You get the benefit of built in debugging/analysis/testing tools along with the ability to view architecture diagrams built by somebody running Ultimate.
If that sounds like you, then it'd be worth it.
Personally, it's Ultimate that I really want.
Ultimate
The closest analog to Ultimate is Visual Studio 2008 Team Edition. It gives you Premium plus a whole ton of Architecture, Testing, and Management tools. The real benefits that I see in Ultimate is that you get IntelliTrace and the Architecture Tools.
If you're working on a large application in a large team, IntelliTrace could be worth the money alone. It allows your testers to run their tests against a debug version of the application. IntelliTrace will then record the actions (even video of the UI if you want) and the Stack Trace of the entire run of the application.
After the recording happens, you as a developer can go back and reply the entire run...viewing the Stack Trace as you go. That allows you to look in to the application and see EXACTLY what was happening when the tester was running your application which means...
No more developers saying "Can Not Reproduce..."!
As for the Architecture tools, you finally have the ability to create Architecture diagrams (Class and Sequence diagrams were used heavily in the demo I saw) and generate your code from the diagrams.
You also get the ability to reverse engineer code back into UML diagrams. This may not sound exciting, but there's another feature that makes it all worth it which is...
The best thing that I saw with the Architecture tools, is the ability to define Architecure rules while you're desiging your diagrams. Once those rules are defined, you can make the part of the build process. Visual Studio will then validate the code against the diagrams each time a build happens. If a developer writes code that violates your architecture rules, BAM! no build.
...Sweet stuff in my opinion.