Buy which ever is cheapest. I think Macmall is usally cheaper then even the student discount.
Only time to buy directly from Apple is when your positive a new version of the mac you want is coming out. This one came out in March so I am 85% sure it won't be updated till next year. I have the new air and it's awesome !!!
My dads birthday is at the start of October and I knew a Mac show was happening a week or 2 later. I thought it was to update the iMacs but could have been ipads as well. My brother and sister made me buy him the iPad 3 so he had something to open on his birthday. I got it at microcenter to save 50 or $100 bucks. I really can't remember and the new ipad came out 2 weeks later. He didn't really care but I did.
My friend did the same thing but got it directly from Apple. Apple let him return the ipad and got the ipad 4 ......