Here you go...
Ridge Racer 6:
Downtown Rave City
Rave City Riverfront
Ridge Racer 7:
Every track from RR6 plus
Seaside Route 765
Ridge Racer (Vita):
Currently available DLC tracks
Oceanfront Cruise Way (from RR3D)
Silver Mountain Skyway (from RR3D)
Redstone Thunder Road (from RR3D)
Yep, you only get three tracks out of the box on the Vita, the rest has to be paid for extra, and even then it doesn't come close to the amount of the previous games. Also, as you can see, the few tracks that are there are all just rehashes with not a single truely original one.
RR7 is by far the best deal out of those three entries imo. Biggest track and car selection, lots of tweaks and additions to the gameplay from RR6, the deepest customization system, etc.
And it can be found for very cheap these days.
There's some small visual changes from RR6 to RR7 of course. 7's graphics are slightly better, textures are sharper, and the color palette in general seems a lot more vivid. Some of the daytimes for certain tracks are different as well (Lakeshore Drive in RR6 for example is raced during the early morning, in RR7 it's during sunset, and so forth). But there's no changes to the course layout itself or stuff like that.
Same for the Vita version, tracks have very minor graphical updates to their surroundings (some for the better, some for the worse...) like a couple additional rock formations on Harborline 765...but you have to be really into the games to even notice those things to begin with. Overall the console games still look better, and they run with a constant 60fps instead of 30.