i'd always substitute first. reason why is because if they attack you, you know what attack they are probably going to use next (the same move) so you can switch to counter that. If they use a non attacking move, like thunder wave on the first turn, it will do nothing to you, so you can then use quiver dance as many times as you feel necessary before attacking. thats the kind of strategy i used with my subs garchomp.
if you know you are slower, you'll have to predict what the opponent will use. he will likely deal damage enough as to render subs useless, as its defences, especially physical defence are pretty frail. if you know they have a super effective attack on you, and they are faster, e.g. aerodactyl, change out.
or if not, attack them and hope for the best.