We are moving up to KA junior. Looks like the OTKs are the way to go but I’m confused by all the different brands. Some say they are all the same, and some say they all have their differences.
Also, I know there is a lot of variation for motor to motor. How do I get a good one?
Pick your favorite color! Just a marketing thing.
The Gillards look super nice. One of those with some custom stickers 
All OTKs are the same, regardless of brand or color. Zero differences. With OTK it should be easy to find parts and support, but make sure you are able to do so. No point in choosing the most “popular” kart if you don’t know how to use it.
Motors are all pretty close, not a big difference, don’t sweat it.
Like TJ said, all the motors are pretty close until you’re searching for that extra 0.01 second. I have 2 motors, and have tested them on the same kart, same day, with 3 different gear ratios, and the RPM curves were almost identical. One motor was purchased as a box stock, the other was a SuperNats prepped motor.
I tell all my new families it’s like sticking your hand in a bag of M&M’s and picking out your favorite color. At the core they all are the same and do the same thing but with a different crunchy coating.
All parts are 100% interchangeable except the graphics kits if you are picky.
I believe any racing chassis can be competitive, what matters is the support you have either at the track or locally. While you should always have spares being able to buy parts and get advice is likely more valuable to the consumer.
As for the motor…TJ gave this advice on another thread and I think its the best advice I have read…