Whether whatever you buy will last you 3 years is going to be largely dependent on what you intend to do with it and how much you want to spend.
The MBP can be substantially more powerful than a MBA. It's also to beef up a MBA to the point that it is more powerful than the low end of the MBP line.
The first question is, what do you need to do? What kind of work? Do you need to run specialized applications? Will you be doing work that involves a lot of heavy lifting? Video editing, heavy photo editing, etc? If not, then the power of one over the other starts to become less of an issue.
How portable do you need it to be? An 11" MBA comes in at just about 2.3 pounds. The 13" is 2.9 pounds. The new MacBook will weigh in at just 2 Pounds.
Do you need (or want) a Retina display? For some, that's a deal breaker. The display on the MBA is good for what it is. I have one. Just bought it in fact. It's a good display. Is it drop dead gorgeous, like a retina display? No, but I don't need that. I was willing to make the trade off to save money and weight.
If you'll be doing standard 'office' type stuff.... MS Office or similar for writing papers, Web for research, email, etc, then any of them would be just fine. The processor in the new MB isn't quite as bad as the early nay-sayers would seem to indicate. Performance tests were largely run on a design from another manufacturer that was defective out of the gate. I believe Apple won't have that problem.
The issue with the new MB (for me) is price. I was able to get a refurbished 11" 2014 MBA for $679. That's almost half the price of the new MB. Don't overlook the deals to be had in the refurb store!
