I have the newer Macbook Pro 13" (retina & solid state) which is substantially lighter and more portable than previous Macbook Pros. While I typically work at the same area most of the time, it being light weight is definitely a bonus.

An air is even lighter and thinner.

I'd say compared to the a 13" Pro, the Air is noticeably lighter but not enough to be a deal-breaker if you value the Retina. For me personally, I was sold on the 2013 Air because of the battery life mostly. There's rumors of major changes to the Air in 2014, seeing as it hasn't changed in appearance since its release, so if you're willing to wait a bit longer it might be worth it.