Both are used from 2013 and are available at a good price and are supposed to last me 4 years only difference is that the air has an ssd and the pro has an hdd neither have a retina display
Is there no difference in specs besides the SSD vs HDD? If not I’d say go with the air. All things considered the SSD makes the computer longer lasting.
Keep in mind that these computers are already 4 years old. Macs do last a long time, but you may have to get creative to keep either of these usable until 2021. Depending on your budget you could probably find a new computer (probably a PC) that’ll serve you better in the long run.