1. Yes, sooooooooooo much more freedom with android, you can do whatever you want with android
2. the best android options are GS 4 and HTC one and your windows mobile option is the lumia 920, of course the iphone 5 is an option if you are happy with how iOS works
3. You might struggle to comfortably use the HTC one and even more so the GS 4 due to the huge increase in screen size, however, it all depends on your hand size
4. Not sure about the GS 4 but with the HTC one, you can transfer all your stuff easily
About setting the htc one up:
http://blog.htc.com/2013/03/htc-one-get-started/
5. Hard to say if you will like android, from what you have said there, you may find touchwiz/GS 4 a bit "too much" as they have crammed a lot into it and the entire thing just feels disjointed where as sense is more minimal and easier too use with a great unified feel throughout. It will take some getting use to, so don't expect to fully appreciate android within 1-2 days
As for GS 4 VS HTC one, they are both very good and in the end, it comes down to personal preference:
GS 4 main pros over the HTC one:
- SD slot
- removable battery
- RGB NLED
- 13MP (important if you crop/zoom in a lot, print photos out on large paper, have a very high res. monitor), better in bright landscape conditions
- some nice hardware features i.e. air view, being able to use the screen with gloves on
HTC one main pros over the GS 4:
- perceived quality
- camera overall (imo)
It might not take as good shots as the GS 4 when it comes to bright landscape scenarios and the HTC one has an ISO bug currently that makes a huge difference so once it is fixed amongst further improvements, the difference will be further reduced.
HTC one is better in low light/dark/indoor conditions. The one has a very quick shutter speed thus you have a better chance of getting a photo that actually has very little blur (a lot of review sites use tri-pods which help immensely) and also means you don't need as steady hands as you would with the GS 3, GS 4 etc. (this is a problem with phones when you choose to use HDR/night mode). Haven't seen many macro shots of the GS 4, but the HTC one is producing some of the best macro shots that I have seen from a phone. You also have a much wider FOV. And then the video recording is excellent overall especially in low light/dark conditions, smoother, more detailed and better colours etc.
The camera will improve further too as mentioned, there is an ISO bug, sharpening is too aggressive and the tech. is very new to HTC (only two phones currently use OIS), so it will improve with updates (nokia have brought two updates to the 920 [similar tech.] that made a huge difference) i.e.
http://www.androidpit.de/htc-one-update-fotos. A lot of the early review sites got pre-production units with early software on them hence why results were poor)
Audio recording is better too (mainly noticeable in loud environments), however, HTC have to replace the mic. tech with something else since it is nokia's tech, this shouldn't happen for a month or so though and they have said that they will replace it with something at least as good, if not better (doubt it though

).....
- overall a better front camera
- speakers, not just in terms of loudness but the placement and quality
Performance is more or less the same although anandtech reported throttling with the GS 4, which will affect games only/mainly and the majority of review sites have said that the one feels marginally faster and smoother, GS 4 has micro lag when opening apps supposedly, of course this could/will improve with updates hopefully....
Battery life looks to be more or less the same, need to see user reviews of the GS 4 first though. Battery benchmarks have very little reflection on real world usage.
Software (you choose to either have a feature full experience all round i.e. GS 4 or one with a nice/simple/refined, less bloated UI all round i.e. htc one), screen (SAMOLED 4.9" [not 5"!!) VS 4.7" LCD) and design are personal preference.
If you check in the GS 4 thread you will see a lot of reviews posted recently and there are plenty of HTC one reviews too.
The best reviews are by anandtech and android central.