I started with PC sim racing. I did everything there was to do, including iRacing, before I ever touched GT5 online.
For some people it's more about the community than the driving, or multiplayer depth. Myself, I could never really get into the iRacing community for whatever reason, I never really set out to try and be the best, or super fast, I just wanted a good group of people to hang out and race with and I found that more easily on GT5 than I ever could on iRacing. It's whatever drives you to do online racing that'll really lead you onto different titles. No one title can cover all the bases, so you stick with what you like for whatever reason you like it for.
I'm more into AC because of how great the driving felt in FVA and nK, but I won't stick around for it. I get bored after a while of hot lapping, I need competition. I grew into that while playing GT5.
But I stuck to GT5 because I ended up becoming friends with a bunch of quick dudes that I liked learning from, it was a great experience - that and GTA. When I grew into this competitive mindset, I was too hungry to stay on GT5, so I tried iRacing again, but I was put off by the community again, and investment of time. So I decided to stick to GT5 until I found something worthwhile on PC again, that I knew would have a big multiplayer community, and that's AC. No guaranteeing how "good" the community will be for me, but I have a lot of my friends from GT5 going over, so I don't think it'll be a problem.