Im planning on getting the Macbook Air 128gig + 4 gig RAM since it is within my budget, but would I be able to upgrade its storage space/RAM later on after buying it or no?
You can't - the drive and RAM are soldiered on to the motherboard, so you can't upgrade after the fact unfortunately. I'd recommend getting the 256gb/8gb model - but it's obviously easy (and cheap) enough to add as much storage as you can desire using an external hard drive.
I got a Macbook Pro because it had some superior specifications that I can't recall, but I got a lot of the stuff upgraded. What the Apple guy told me was that they generally recommend purchasing computers with lower specifications and replacing them every few years. Yes, I paid through the nose (well actually my scholarship money paid for it lol) but I have no intentions on replacing it during my 7 years at university at the bare minimum.
I've got a 11' Macbook Air (the stock 256 gb model,) and a 15' Pro. Honestly, I find myself using the Air much more - for such a lightweight laptop it's got surprisingly good performance, it's great for taking places (much more portable compared to the relatively bulky Pro!) I'm still glad I've got the Pro, but during the uni holidays I find that it sits unused most of the time (when I don't need a big screen to write things or read a million papers.)
The main benefit of the Pro over the Air is it's screen, the Retina screen is a really fantastic display - by far the best display I've ever seen on a laptop.
In retrospect, I probably should have got the fully upgraded 11' Air and buy a decent external display.
The 'buy a decent computer, sell often,' works well if you're willing to go through the hassle of selling your computer every few years and buying a new one. Macs hold their value quite well (it's not uncommon to sell for 75% of the purchase price.)