Help me pick my first Mac

Sorry, I know these threads are everywhere. Still, I would appreciate some advice.

I'm starting med school in the fall. My parents are willing to throw down for a new laptop for me. Since its their money, I have a decent amount of leeway; that said, I don't want to be an ****** and buy something I don't need.

I'm between the base 13' and the base 15'.

I would use it for med school stuff (honestly not even all that sure what that entails) and light gaming (say skyrim, diablo 3, baldurs gate enhanced). I'm not a huge gamer, but I would like to have the option to play from time to time. I am happy running the games on mediocre resolutions, but I want decent fps and don't want the graphics to look like absolute garbage.

I like portability, since I'll be lugging it around a hospital/school. I do have an iPad, though. How portable is the 15'? I know the 13' would be sweet in that regard.

Please don't suggest either of the airs. Whichever pro I get, I'll be throwing in a ram upgrade and an ssd upgrade within the next year. Same for the retina. Upgrade ability matters.

I need the laptop to be relevant for the next four years, as I will not be getting a new one until I graduate.

So, is the extra 500 dollars worth the extra processing power (quad core) and the graphics card, do I need the graphics card, how big is the portability trade off, how helpful is the extra screen real estate, etc. What do the boards Mac experts recommend?

I apologize if some of these concerns are basic, but this will be my first ever Mac! I'm pretty sure I have covered all the relevant info to not get flamed.

Cheers and thanks!