On one hand the larger screen will be much useful when you are trying to get work done. On the other hand the 13 will be easier to move around. Btw don't be one of those people who brings a laptop to engineering class for notes. You'll never actually use it for notes, just distraction . There are too many diagrams and figures for taking notes on a laptop. So don't expect to get much use out of the laptop. The only time a laptop's useful is when you need to collaborate with people. But even then it might not be as useful as you think.
To be really honest. I never had much use for a laptop while I was in undergrad and in grad school however it was much more useful. But the situations were completely different. My supervisor never gave me a good desktop and my laptop allowed me to keep the simulator I was working on with me at all times. So If I felt tired of the lab I can pack up go home and work late into the night. Otherwise to be completely honest I think that a laptop was never that useful.
But if you must have a laptop remember that your money situation is only going to become more tight unless 1) you some how manage to get a job while studying (big if, grades are important and now a days getting a grad degree is almost a requirement for a good engineering job and good grades are necessary for that) or 2) Bank of mom is generous. So beside whether the screen is big enough on the mac you should also consider whether it will last you the 4 years and if not will you be able to replace it between now and then? I know macs are durable but the battery that can still crap out on you in the 4 years. Accidents can happen like coffee spills or if that hot girl you are doing a lab with jumps you while you guys are working on something in your room and knocks your mac to the ground..
It might be useful to think a bit more about whether or not the costly mac option is the best for you.
