I looked around a bit for this, but couldn't find much on Caravan Guard other than the official FAQ. The simple situation is reacting to Spy with Caravan Guard. The question is whether I draw a card before or after my opponent chooses what to do with my top card.
I can rationalize all 3 choices. Playing an action could always have the same speed, where the Spy was played first and get's to finish first, and the Caravan Guard was played second so it finishes second. Using the Reaction could make it faster, causing you to draw before being Spied on - I hate drawing corollaries to Magic, but cards played in succession resolve in reverse order. Playing on-play could effectively make both effects happen at the same time and you would get to choose which happens first, but then who would choose which happens first?
It doesn't seem like a profound question, but with the number of Spy variants, it could be a useful thing to have an answer to.