both apple products are awesome. and both can more than likely be plugged into his aftermarket stereo if hes got one.
this is a very pricey install but there are a lot cheaper ways to do it just check youtube.
My brother is a fashionable boy who are willing to have an smartphone or portable Tablet PC which can meet his requirements: surf the internet fluently, play games freely, touch screen, use e-mail and other functions, he want to buy an iphone 4s(and know he has already have a very good Nokia) so i want to buy an ipad 3 for him... but i have little idea on these devices...Do you think which is better? thank you very much!