English as a second language

If anyone has questions about English, or maybe someone who speaks it natively but doesn't understand the mechanics, then feel free to ask questions here.

For the add-on to make something a noun, '-or' and '-er', which is right to use? e.g. Disruptor and Disrupter
Is there a difference between those two add-ons? Is maybe one of them a British add-on and one an American add-on?

I think the first one is technically the right one but the 2nd one is how you pronounce it, at least in America.  There is no change in meaning though.