should i replace my macbook pro 15(early 2011) with macbook air 13 (2013)?

I have a macbook pro 15 (early 2011) basic model with 4gb ram and 2ghz quad-core i7. I am trying to replace it with the newly released macbook air because of its portability and light weight. I do not own any other computer so whatever I decide, it will be my primary computer. I do web surfing, youtube, school assignments (word, excel, etc.) and very simple/minimal/amateur video watching, editing, and photo editing (aperture, photoshop). I don't care about the storage since i intend to purchase an external hard drive but am willing to upgrade my ram and cpu if i buy MBA 13. However I'm conflicting because MBA seems like it will become slower and quite useless after a few years. What should I do? is there any other options I should think about like wait and get more money for macbook pro retina?

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