The Kit and the Race Roller are the same. The roller comes pre assembled obviously and when I bought my second 8ight I got the roller so I wouldn't have to build it - the factory actually does a good job building the cars.
The roller is suppose to come with a easy to drive set-up, but if you build the kit you can build whatever set up you want.
Big price difference? where are you looking and which is cheaper?
If you have never built and 1/8th scale before I would suggest the kit, but if you can save money with the roller go for it. it still comes with detailed instructions.