This may seem like a silly question but i'll ask anyway. Whats the difference between a 10 amp brushless speed control for speed 400 and below motors compared to a 20 amp brushless speed control for speed 400 and below as well.?

I forgot to mention. also, how do you know how much amp you should use for certain electric brusheless motor?

I always thought that Speed 400 motors referred to the brushed can motors, but if it's just the amperage you're wondering about, then really the only difference is the cost and the amount of current the ESC can handle before it self destructs. There may be a difference in the BEC circuit too, depending on the manufacture, providing more power for the servos.

Which one you need depends on a lot of things. Your battery, your gearing and your prop. Generally, the manufacture will recommend an ESC of a certain size. If they don't, you can always go to a site like this:

http://brantuas.com/ezcalc/dma1.asp

and plug in your details to get an idea of the amps your motor/prop/battery will pull at full throttle. Then buy an ESC that is a bigger as these programs are not 100% accurate.

I'm assuming that you already know that brushed & brushless speed controllers can only be used with motors of the same type.