The MBP with 8 GB Ram and SSD will give you 4 years. The Air is limited to 4 GB of ram, which might limit its longevity.
But, if I was limited to getting a 13'' Macbook, I might still go for the air. There is not much in the 13'' MBP that justifies carrying around 1.5 extra pounds, the screen resolution is higher on the Air, and once you factor in the cost for the SSD, the price difference is small.
I disagree...with the SSD in the MBA, even IF you bumped in to the RAM limitations (I have both 2010 and 2011 11" Airs), the SSD page in/out performance makes the lack of RAM for MOST applications, especially now, a non-issue. I routinely use my 11 and 13" 2011 MBAirs for post production, editing and on location recording with Logic, Ableton and Audition. I also use one of the new Lacie ThunderBolt 1 TB drives for storage in the field. These machines are flat out FireCrackers! The SSD makes up for any RAM shortcomings with today, as well as the near future applications. Keep in mind, most apps are just recently (or not quite yet) moving to 64 bit platforms and finally able to take advantage of more than 2-3gigs of RAM. You'll be able to use it for at least 4 more years.
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An air would be a step back in performance. My suggestion is since you already have a 2011 model wait for the next MBA models to make the switch. There is absolutely no reason to doubt it will have 8gb optional which is important for a machine that has RAM soldiered to the board. The only reason I would make a switch like that is if money is really no issue (not the case for me).
Here is an excellent chart using GeekBench to show you the "Step back in performance by using an Air vs. a 13" Pro 2.7.
http://www.barefeats.com/mba11_04.html
Notice the improvement of the Airs over last year's Core2Duo offerings. While indeed the Pro is a 10-15% CPU performance boost over the Air...when you take in to account the benefits of the SSD...the lightness, the thin profile, and what you TRULY need...it's a compelling option. I owned a couple of 13s over the years. Last year, I picked my wife up the 11" 2010 1.6/4gig RAM/128 SSD and fell in love! This year, I picked up an 11" ultimate and 13" i5 with 256ssd for our field work. I also own a 17" 2011 MBP...but when it comes time to portability...I can carry external drives and power cords...Air cards and the Air Super drive...WITH both 11 and 13" airs for the same weight penalty as a single 13" Pro with the trimmings
We do a LOT of walking/hiking/climbing for shots...so any ounces saved are huge...and the option to have two computers (redundancy) when we are 250 miles from civilization is a dream come true
Thanks all for the replies!
Definitely a lot to think about here. One thing I'd like to mention is that I basically do nothing taxing on this machine. I havent run Ableton in about 8 months or so, and even then its just to fiddle. I buy all my movies and music from iTunes so transcoding isn't an issue. Honestly, I'm not far from only needing an iPad. Other than very light Adobe Illustrator work (which i caould do at work anyway), it serves movies to my ATV2.
In fact 90% of what this machine does is serve movies.
I had been pondering either an 11" or 13" with 128 SSD. Again, I now carry it everyday with me, when I bought the pro it rarely left the house.
You answered the question. For YOU, the Air is a perfect option. Not that you CAN'T pick up Ableton whenever you want...it runs FAST on an Air

In fact, the software available today...other than gaming software; which will have the same performance essentially on the 13" pro due to the integrated Intel 3000 GPU....will run JUST Fine on the Air line-up. With so much attention by Apple being paid to the iCloud services...local storage will become less and less important...where as mobile comm will become more essential (tethering, 3g radios built in, Air Cards, MiFi, et al).
Our audio library in iTunes is well over 105,000 tracks. Not all music--Game show audio, presidential speeches, different zings, bangs, and drum rolls...I'm unable to take advantage of iTunes match at this point. However...I've found the external Lacie solutions to be indispensable. Especially ThunderBolt connections. I definitely hope prices continue to come down on the TB peripherals...but for now, time is money in my line of work...and redundancy in storage is paramount.
YMMV
j