My WGT is a BMI. My 1/12th used to be a BMI as well but they haven't had a new car come out in awhile so I switched to CRC. It is a great car...much better than the driver
To be honest though all the current 1/12th scale cars are top notch so whichever one has the options you want/like will be a good one. I do have a spare WGT and 1/12th if you want to try one. The WGT is the BMI I had before my current one. The 1/12th is the previous CRC chassis. The WGT is a lot closer to being runable...it could use new front bearings but they are runable. The 1/12th needs a lower rear pod plate. Both need tires. I do have a set of pretty worn out WGT tires that will work but they are a bit chewed up and trued down to carpet size so they won't last more than a race day I don't think.
As for the differences between the classes...both use the same motor and battery so the WGT being a bigger car will be slower. 1/12th runs prototype style bodies (GTP, LMP cars) so they have better down force. WGT runs front engine, rear wheel drive, GT car bodies. As for tires 1/12th you can use any tire compounds you like and they tend to be a little less expensive than WGT tires. WGT runs a spec compound (lilac) that you can get from Jaco, BSR, or CRC. Lilac is a pretty hard compound so tire wear is usually better in WGT than in 1/12th but because you don't have a choice of compounds and the bodies have less down force they tend to be a little trickier to get a good setup working. That said it also takes away having to hunt for the right compounds like we do in 1/12th. Once you do get the WGT to handle well it is easier to drive than the 1/12th due to its larger size and lower speeds.
1/12th is a more established class at the track with more drivers, when they show up...WGT we only have 3 of us for now so it remains to be seen if the class will prosper or fail.
So that's pretty much the best info I can give you to help you make a choice. Let me know if you have more questions.