How many corporations are developing Windows based apps these days? I would think the vast majority of in-house apps today are browser based. Along with online browser apps, that's essentially what I've been doing for the past 15 years, and it's gotten old.
I'm a little more interested in developing shrink-wrapped applications. Perhaps some shareware, perhaps get involved with some open source projects.
A lot as Leros said. I've done both in different roles, webapp and local. It really depends on the need. I'd probably make something local if it wasn't used by a ton of people or needed to be reached by a lot of employees.
We have more advanced local apps that do some stuff that webapps aren't suitable for (software upgrades for example). Basic text input, stuff that would've been done with paper files years ago are webapps, at least the ones I've created.
I wouldn't start on .Net stuff today though, local or webapps. Unless its for financial incentive for a job, there's no shortage of MS stack jobs and there's nothing wrong with them or that fact. There's also no shortage of PHP jobs and nothing wrong with that either.
I use Mono (not much, but it's free). Why not install MonoDevelop and get your feet wet. Honestly, I prefer it for a lighter weight install than VS. Seems everytime I install VS there's no getting rid of it and it's heavy as can be. It's just invasive, its screwed up Windows installs of mine before. While its the best IDE out there, I just don't like it unless its on a company provided machine.
Since GUI apps are becoming more and more legacy apps in the age of webapps, Python should be fine. If you need more performance (Photoshop-level, something hardcore), forget .Net/C#, go straight to C++ and be done with it.
I'd like to expand on this comment. Instead of getting into C++ for a GUI app, I'd prefer to use Python + Cython, you'll still get the performance increase. Not sure on the limitations to Cython vs working on C++ but it's where I'd look. That's just me, it's always easier to use what you already know.
Interesting that the top five languages on that chart are all C or derivatives of C.
If C# is dropping in popularity (I don't know how they come up with those numbers) then it is likely due to the shift toward mobile platforms and Microsoft's lack of a viable solution in that realm.
I'd guess it's because the mobile realm has no dominant player other than Objective-C, and since they won't use that, they adopted HTML5+JS. Brings in a lot more guys than just the C# crowd. No one's corporate backed language besides Obj-C is really taking off. In fact, unless the OP wants a corporate-drone job hacking away on C# GUI apps all day, ObjectiveC would probably be the better investment of time and effort.
Getting into ObjC, C# on Mono or Python costs nothing and 1st class apps are written with all 3. Paying to develop seems like a huge favor to company X both upfront, then with the dependence / lock-in later on, not to mention the validation you give their pay-to-develop platform.