"why not just do one or the other?"
Pure ex post is boring. Just get a list of players, order it by your favorite measure of value, and then give Trout a bonus of however much your favorite projection system puts him at. (With some tweaks for possible ranges of outcome, but whatever tweaks you put on it, Trout + projection-for-the-rest-of-his-career isn't going to be a contender here.)
Pure ex ante is hard to do (since we don't, that I know of, have scouting reports for a young Hank Aaron), but also unnecessarily restrictive. We know that Hank Aaron is capable of having the career that he, in fact, had. Why discard that information?
But perhaps more to the point: it's just a game, and you can play it however you want. If you want to project players just based on their amateur scouting reports, fine, that's one way to play the game. The OP didn't give us much guidance on how this game was to be played, so I picked one way to play it that sounded like fun to me. There are lots of other ways to do it too.
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