Which is more important in air travel luggage? Durability or lightness?  

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Light could be an ultra thin plastic bag...but it might not last one trip.Very durable ...light weight, haliburton but it bangs up badly.

Durability, absolutely!
Have you ever SEEN what the baggage handlers do to luggage???? Horrifying. Remember the Samsonite TV ad with the gorilla? That about does it.
My hubby did a project for Homeland Security that had him working in the baggage screening area for a few weeks. He was absolutely shocked at how badly luggage was treated. He told me of one time when a handler dropped a bag and rather than picking it up he just pushed it along with the front of the baggage vehicle. Tire markes and road rash all over the thing and nobody cared.
I'll tolerate the heavier stronger luggage, thank you.

Durable. Making lightweight...
Durable. Making lightweight baggage that will ruin only makes more landfill. Making lightweight luggage only pays tribute the populations increasing sense of laziness. Safely transporting your belongings and getting some much needed exercise far outweighs the benefits of light luggage. Besides, most of the weight comes from what's inside the luggage not the bag itself. How much difference does it really make?
If you want lighter luggage, then pack less of your belongings. DO you really need three pairs of boots? Are you really going to read 5 books at the beach? Maybe get an MP3 player rather than taking your CD book....

Requirements
1. Durable
2. Any color except black
3. Affordable, but not cheap
I have found that the expensive stuff gets just as banged up as the medium-expensive stuff, so I go for the medium. The cheap stuff falls apart immediately, the handle mechanism and the zippers always break at the most inopportune moment, usually on their maiden voyage.

Lightweight.
for the same reasons Whitespike chose "durable", oddly enough.
If you pack light, and use carry-on exclusively, gorilla-proof durability isn't an issue.
A light bag can be slung over your shoulder so you can actually CARRY it-- down the stairs to the subway and up again... onto the bus and off again... through the airport... onto the plane and off again.
I refuse to be one of those public menaces who drag their wheeled luggage oh so slowly through the airport... stopping short... unpredictably changing course... standing stationary instead of walking, on moving WALKways... flummoxed by a few measly stairs. (Can you tell I suffer from road rage?)
I've been using the same light weight Land's End duffle since I was in school (that's a LONG time) and it's not a candidate for the landfill, yet.

Carry-on isn't an option very often these days
I'm sorry but there is no way I can go away for a week and manage on 3oz of shampoo and conditioner. And No, don't tell me to use what the hotel offers. My fine, curly hair would be a rats nest after one washing. I've begun leaving full kits of toiletries at my sister's house and our temporary New Mexico location so that I can just carry on my clothes. But if I go anywhere else I'm stuck with checked baggage.
I'm also not entirely sure that lightweight and durable can't be found in the same product. I agree with Riki that there is a medium price range that gives the best of both for the most reasonable price. I have a Wenger carry-on duffle that I've been using a long time that has held up quite nicely. I also have a small Samsonite that has had some longevity as well. But he big bags that get checked in, boy, do those things ever have to survive a beating. I doubt any othem at no matter what price would with stand years of that kind of abuse.
I suppose that I could just get some hard aluminum cases...but portable is not a word I'd apply to those things no matter how much exercise I'm willing to get!