The communities for SM and FFR are kind of blended together a bit, people tend to play both. That being said, this is the FFR Forums so you're bound to see a bit more FFR discussion. Typically the brag board gets about an equal amount of score posts for each game so I would argue that StepMania might actually be slightly more popular overall.
The other main difference is that you don't have to download anything for FFR. Its easy to just load up and play it.
For StepMania, I would suggest using 5. Its newer, has a few less bugs, and looks a little prettier. Generally speaking the people who use 3.9 either are used to it and don't want to switch or have bad computers that makes 5 feel worse.
In terms of how you should start out, you should first understand that people generally play here with a 4 key setup, i.e. 'z','x','.','/'. You play with 2 hands and use your index and middle fingers. Pretty straight forward here.
Generally StepMania is focused on downloading packs of files. There is no good way to go about downloading single files, you just download a pack with usually 40-90 files or so on average and usually get some sort of quality assurance through that. You should start by looking at the packs mentioned in this thread:
http://www.flashflashrevolution.com/...d.php?t=133223
You should look for the ones with stars next to their name as they will likely be a bit more novice friendly.
Welcome to StepMania/FFR!