For position players it really depends what you are looking for. Power ratings are fairly consistent to performance but you often need to have reasonable contact numbers to go along with that. Players who hit both lefties and righties above 70 ratings are usually money but, again, other factors matter. Look for a rating above 90 in range and fielding if you value SS and CF to anchor the middle of your defense. Pitch calling is an important factor for catchers, as it can help your entire rotation. The key for you with regard to prospects is to know that (1) they grow by far the most in the first 3 years and (2) most players do not advance much after 4 years and (3) the great majority of prospects will not reach their fully rated potential. Of course, the amount that you invest in training in budgeting influences all of this, so you want to invest heavily there if you want to develop your players.