I worked as a computer repair tech in the past.
Screen size is pretty irrelevant, with the current form factor (including size and weight) and performance of computers.
When it comes to screens, I would focus more on screen resolution and how easy it is to read on a laptop. Since all of your books, at least from MS-1 to MS-4 probably have digitalized text, you will probably be doing a lot of reading on your iPad/Laptop. There is a positive correlation between screen resolution and how fast you can read. Obviously the faster you can read the material the better off you are in medical school.
So, a retina display would be advantageous in that respect. If you get an iPad, get a retina display. There is no reason not to. It is so much easier to read on it.
I have seen so many crappy displays on expensive $1000+ HPs and Dells while setting up brand new computers, so be careful. A "nice screen" is a more subjective feature that should be paid attention to, once the technical specifications of the computer are deemed as acceptable.
Other more subjective features include form factor (size and weight) and battery life. Obviously portability is going to be essential in medical school, so form factor is a big deal. Battery life has to be "acceptable", but make sure you know how to set "power consumption" features on any operating system that you are going to use.
Laptop failure rates by brand
http://www.squaretrade.com/htm/pdf/SquareTrade_laptop_reliability_1109.pdf (See page 6)
Needless to say, I don't recommend getting an HP or a Gateway/Acer (same manufacturer, different brands).
When I was working in tech, about 1.5 years ago, ASUS computers were the best value for the dollar, when it can to performance (actual hardware). It may be different now, but keep this in mind. I would not worry about being "cheap", just get whatever would make you happy and what you would want to work with.
Also, if you break the screen/LCD fails or the motherboard fails, the value of the repair will likely exceed the value of the laptop. So keep this in mind.
Any warranty on a laptop from Apple does not cover accidental damage, which makes the warranty fairly useless. Best Buy warranties (where I used to work at) are not favorable to med students, even if they cover accidental damage. If your laptop needs warranty service from Best Buy/Geek Squad, it HAS to go to the service center, which means you will be without a laptop for 2 weeks.
If you chose to get a warranty, Squaretrade is probably your best bet. A warranty on a laptop (and iPads, for that matter) can end up pretty useless without accidental damage, so keep this in mind. DC jacks (where the power charger connects to the computer) shifting/moving around, keys falling off a keyboard, are all considered accidental damage in the computer repair industry.
Writing with an iPad can be a challenge, for some people. There is no place to rest your wrist, so writing can be awkward. I have a case with a huge bezel (that I can rest my wrist on) so writing is not challenging and or awkward. It is called the Lifepoof nuud case. It is expensive, but worth it. You can get it at Best Buy, test it out, and if for some reason it doesn't work out, you can return it.
For a stylus, I use to Pogo Connect pen. I like it, although there may be a better stylus out there than when I purchased it in December 2012. It is similar to writing on a dry erase board.
Finding an app that works well for taking notes may take some time. I use an app that allows me to zoom in and out. It also syncs with Dropbox and other cloud storage.
You have to practice writing on an iPad before ever taking notes in class. You cannot just break out your iPad and stylus for the first time in class and expect to take notes proficiently, no matter how ideal your setup is.
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I also have a multifunction printer with a scanner (Canon MX892). Since I am an undergrad and not all of my books are in digitalized text, I sometimes have to scan my books. I do it about a week before the semester starts.
I get the books cheaply from wherever. Once they come in the mail, I go to Kinko's to get the spines cut off the books for like $1 each.
I have my scanner set on a desk and I just watch movies while scanning the book, page by page, on the platen. I scan in 600 DPI (in color, if the book is in color) and directly in to PDF (my scanner offers this) on to a flash drive. I scan about 10 pages at a time per file and then combine them later using Adobe Acrobat Pro on a computer. I only use Adobe to combine the files, because the scans turn out so well.
Scanning the first 100 pages can be arduous, but after that, you get used to it.
I do not use the automatic document feeder because the pages can end up skewed or the pages can get jammed in the feeder. Also, the quality is much higher when scanned on the platen.
Get the ink cheaply off of ebay.
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If you get a Macbook Air, make sure to upgrade the RAM before you purchase the computer, because if you don't, you will never be able to upgrade the RAM in the future. There are a very limited amount of slots for RAM on the Air. This is not the case with the Macbook Pro. Keep this in mind.
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Anyways, I hate paper and bulk. Although I take virtually all of my notes on my iPad, I always carry a notebook with me. I also do my (engineering) homeworks on paper, which typically takes less time (I make mistakes and I have to erase a lot-harder to do on an iPad).
You may think I am crazy for scanning my books, but I really hate bulk. The less I have to carry, the better.
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I forgot one thing. Get a big, PORTABLE hard drive. It's like $80 at most.
Learn how to do an image back up for whatever operating system you end up with (Windows, OS X). Use Youtube to figure it out.
Although it takes a long time to back up stuff (at least on OS X), it will save you grief if your computer ever were to fail.
Flash drives are suitable for backing up work you have done in the short-term, that you have not backed up yet with the "image back up".
