Big,
You're a better money manager than I am if you plan on getting a return of 15K a year off 125K after taxes. I'd be inclined to ask them to pay over several years to minimize the tax impacts while they are likely to not be making much of an income in the minors.
Other than that I tend to agree. Maturity and the player's goals should be more of a factor in making the decision than $. Some players are more likely to succeed in baseball if they go out of HS, some are more likely if they go to college. If the goal is to play major league baseball then the path most likely to get a player to the big leagues is the criteria to use.