The main difference is at the breakdown, it is the main role of a flanker to be first at the scene of a breakdown and be the first to detach from the scrum. The Number 8 is generally slightly larger than the blindside and also a bit slower. The Number 8 is able to pick and go, and even then still gets more time on the ball.
well playing number 8 entitles you to run with the ball a whole lot more. Think of flank as more on a defending role and that you will need to be the first line of defence and do your best to steal as many balls as you can. number 8 is more of an attacking role and ball carrying position. to get better at number 8 get stronger and faster. work out at the gym (legs and arms) more. do sprint training and youll eventually be the best :D