Where would you put it? Would it be in plain site where anyone and everyone could walk in on him where he was playing? Are all the games for both consoles rated? Discuss with him what kind of games he is interested in. For instance, what is the real purpose of the purchase? Is he interested in the sports games, the exercise games, the board and puzzle type games. Or is he interested in the gun/shooting/ darker side of the coin games? Once you get a picture of what it is all about make a contract with him.
In the agreement decide how much time per night, per week he can use the console. For instance, if he has a test, should he be able to use it the night before the test. Should he do all his homework before he sits down to the console. Does all his chores have to be done before, etc. Also include the consequences if the agreement is broken and write that into the contract. When everything that you both agree to is written in to the contract then you both sign it and go out to purchase the item. Also in the agreement put in a stipulation that the agreement will be re-evaluated and renegotiated in six months. So if you feel it is not working, or your child feels he has proven himself and wants more time or wants more privileges like buying additional games, etc. then at the six month mark, you sit down and listen to each other and adjust the agreement according to the points you are both making and sign it again with the same stipulation that you will re-evaluate and re-negotiate in six months.
In this way you are working together and working towards mutual goals, which is your son gaining the privileges he wants without going behind your back, and without you judging him or putting him down.