I've always heard of the recruiting bonus of players within 70 miles, but how much of an effect does it really have? If I was looking at two top 5 players at their position, one slightly within that bonus and one slightly outside of it, both whom I know I will have to battle for, should that extra bonus be the decision maker or should I just go for the player who I would rather have?

There is no multiplier or additional value added in the 70 mile rule.  The impact is simply that some players that would not consider you if they were further out will now be willing to talk to you.