I have both. A desktop has modularity and you are able to interchange parts, which isn't possible with modern laptops(older laptops used to allow you to change out smaller parts, like your ram or your storage). Laptops give you the ease of transportation. For me, I use my macbook for school and i play games on my macbook. Needing a better graphics card for a desktop results in needing to change it out as games and hardware progresses, but with laptops, I think its best to buy when you are only needing low power tasks(like in my case, schoolwork). In a perfect world, its best to have both!
PS. having a macbook and an iphone combo is great. if you are in the apple ecosystem already, get a macbook, the continuity is wonderful.