The best computer is the one he'll enjoy using. Consider telling him that you'll contribute what you paid for the sibling's computer & he can choose to come up with any additional for the computer he researches & wants. This way it's fair to the sibling & HE gets to choose what to spend (& how to earn it) & what computer to get, Mac or Windows. The slightly older Mac models will come down in price yet be fully warranted as new; I've purchased a previous model (from Amazon) when I preferred its configuration of ports, etc. The Apple Store online also sells refurbished models with a like-new warranty.
Disclosure: I love Macs, detest Windows, & I'm a computer professional. So does our DS, age 24, post graduate engineer. DH sticks with Windows but ends up with more computer issues than we ever do.