The role of flanker and 8man on defence -- chase the ball, make tackles, ruck and (hopefully) generate turnovers -- are much the same. But on offense the 8man has the additional job of (1) controlling the ball in the scrum and (2) occasionally running the ball off of the scrum. Either can be tricky, and a player needs a good feel for the game before he (or she) takes on those roles. Start at flanker. Once you have some experience, then by all means give 8man a try.