I have had a computer at home and at work since 1983. I would never ever go with a custom built one. I would get a name brand, from a reputable name brand dealer.
I also wouldn't get a desk top. You might want to take your computer to class, work somewhere else, or one of a thousand reasons NOT to get a desk top.
Visit with Apple as well if you have a store or call them. That way you would have and be familiar with BOTH platforms and not limit yourself in interacting with students and others.
Currently I have a Mac Air--wouldn't go back at all to the other world.