I had a 2011 MBA like that, except it was an i5. Really liked it, but wouldn't consider it today, certainly not for $600. The 2014 has 802.11ac wifi that is 4 or 5 times faster, USB ports that are probably 10x faster and a battery that approaches twice the run time. Should also have a faster SSD. I doubt that you would notice a lot of difference between the i7 and i5, I currently have a 2013 i7 and it is maybe 20% faster than my 2011 MBA, but part of that is the newer generation chips. The i7 also drains the battery a little faster, but not a big deal. On the 13" MBA, the SD card slot is really slow compared to the newer models, it is connected to the slow USB 2.0 bus.
Of course the downside is the 256gb ssd on the 2011 vs the 128 on the 2014, so that might tip the scales. But if you want a machine to last 5 years, that 2011 would be 10 years old. IMO, that is unrealistic. I stopped using my 2008 MBP after about 5 years, just too slow, big and heavy to be appealing at that point. Am still completely happy with my 2013 MBA at three and a half years, but it won't last forever. I think 6 years is a more reasonable life (for a primary computer, at least) unless your needs are really modest and you don't want to keep updating software.
Also consider battery condition on the 2011, it may be near the end and that could cost somewhere around $120 to replace.
