The pont to ask: will it be your only computer (and do you have a specific pricetag you do not want to bypass)
If not, opt for ease of transport and choose the MBAir
If yes, also look into the MBPro as you will have more choices for screen size
Unfortunately current MBPro models can't be upgraded anymore (I think the last version that allowed so, was the 2012 model), so for both (Pro&Air) you will have to buy a predefined config, which should also meet your future needs. Never mind for the disk: you could add f.i. an external USB or network drive.