I can't give you a break down on the different variations of the letters. Google is your best bet there but it is often how many seats and engine etc.

Both are mid wing designs which will excel at IMAC so you can't go wrong either way. The canopy is different and thats really about it. I have flown both 50cc Extras from Aeroworks and can tell you to pretty much pick which one you like better and have fun. The think the 300 may have an edge but by a narrow margin.

The most important thing is to do some reading and spend the time to set it up properly.. neither will let you down.