1. My copy doesn't have two discs, so I'm not sure about the first point. I wasn't aware any others had more than one disc.
2. The games are very similar. I'd say that the graphics are a little better in TT. They use the same physics engine but TT is probably more realistic than GT4 too, though there are still weird moments, such as the odd-feeling snaking you can get into when you go onto the grass or gravel.
Sound effects are good, better than GT4, either view is good to race from (I use the excellent on-board view). Lots of bikes/scooters to choose from.
Not as much to do as GT4 - and you don't buy the bikes or tuning bits for them - you do races to win each model, and when you win them almost all of them give you the option to fit a sports exhaust. Regardless, all the bikes offer small tuning parameters like final drive or damping. There are series like there are in GT4, and you win bikes from these too. Whenever you win a race bike, it lets you choose your race number.
I think most of GT4's tracks have survived into TT, and you can race at Valencia as well.
And of course, the photomode is carried over, only without the locations - track stuff only: