I'm Canadian. Canadian English, United States English, U.K. English, South African English, Australian English, New Zealand English, and so forth. The nouns, pronouns, adjectives, verbs, adverbs, and on and on... they are the same. I have spoken with people from all of those countries and we simply interact and talk with no problems. The accents are different and there may be some small differences in how we refer to items; for example, "The subway" in America is "The tube" in Britain. In The United States they spell some words differently than British or Canadian; for example, colour (Canada, U.K.) and color (United States). But other than a few small differences, learning English would involve the same challenges.

I think they're both pretty similar to learn. They just sound pretty different. As for me, American English is easier to understand.