For me GT3 was all about trying to master Nurnburgring, perfect my laptime as much as possible. I loved the S Vettel challenge in GT5 with the GT monstercar, and especially Nurnburgring.
I had stopped playing GT for a long time but with GT Sport what I discovered is that the sensation of driving is superb. It feels right. Absolutely amazing. I drive with Thrustmaster wheel, but also with the controller when I am lazy and do not want to set up the wheel. I find that even with standard controller the feel is good.
So if you are looking for events, you will find GT sport to have quite similar content as past incarnations, what I miss is the GT5 story challenges.
If you just want to race your knuckles white on Nurnburgring ans some other awesome locations, GTSport is simply the best.
I have yet to test the online.