Corsairs gives a good summary. A scout who heavily favours tools will focus mostly on Potential ratings when assessing a player. It won't much matter to the tools scout if the player is miles away from achieving his potential. A scout who heavily favours ability will focus more on current ability (current ratings) and age, and project a player based on how guys of that age and ability normally develop. Since we know
OOTP tries to match real life aging curves, and that Potential ratings are pretty volatile, the 'favor ability' philosophy is not such a bad attitude for an
OOTP scout to adopt.
I haven't really had a chance to play
OOTP-9 since beta, so I don't know whether there's a big advantage to one type of scout over another. I'd personally want a tools scout, only because I like the high risk, high reward prospects. In any case, the difference isn't quite as dramatic as the above paragraph might make it sound; any type of scout should at least be playable, and all scouts take, to varying degrees, current ability and Potential ratings into account.