Macbook Air with 8 GB RAM vs Base Macbook Pro with Retina?

Hey guys, great forum, first post here.

I was initially thinking of getting the Macbook Pro without the retina, as the 128 SSD is too low even with it's benefits, but I think I'll go with it, and just buy an external hard drive later on.

My question is which one is better when it comes to performance, and even overall, I may also run Civilization 5 on it, and would also like to see it's overall features considering if I buy the 8 Gig RAM MBA or just the base [4 Gig] rMBP?

Is the MBA with the 8 Gig RAM better than the base rMBP? Might be a silly question but I'd like to know.

Also another silly-ish question, since the 128 SSD is just too low, and upgrading the solid state is pretty pricey, is it convenient to get an external hard drive later on in order to manage space well?

I'll also use to do lots of student work, maybe run windows side by side in the future, (I'll have to manage space somehow) downloading a ton of TV shows/Movies, browsing multiple tabs, applications and all that.

It seems like a tough decision.. for me.

Thanks.

Buying an external hard drive to lug around with either a MBA or retina machine kind of defeats the purpose of having a notebook computer. You really need to consider a larger flash drive in either choice you make. I would say nowadays, 256 GB or there about is the minimum I would choose. And the same goes for the memory. While you could certainly live with 4 GB, you have to think about future use. I would go for 8 GB.

As you can see, that ups the $$ cost of both machines. The disadvantage of buying either one is that they can not be upgraded by you the user. Memory is fixed and in order to swap out the flash drive later on, it will be costly and probably should be done by an authorized Apple technician.

The standard 13" MacBook Pro that Apple still sells is fully upgradeable by you the user and can even accommodate a flash drive in place of the optical drive. It also comes with a 500 GB hard drive.