What is the difference between these words?

Not a whole lot of difference, actually.
Envision means to picture oneself in a certain situation, like you envision yourself in the front row of your favorite concert.

Envisage means to have a mental picture of something before it happens, as well, but maybe not including yourself, like "envisages a completely redecorated living room"

I'm sure somebody will disagree with my examples, but I used the m-w link here to look them up, so don't kill the messenger

From The New Penguin English Dictionary

"envisage or envision? These two words are synonymous in the sense of 'to form a mental picture of (something); esp in advance'... ...envisage is more commonly used in British English; envision in American English."