How should I decide between a new set of All Clad or GreenPan pots and pans?

I have a couple of the All Clad nonstick pans I've picked up at TJ Maxx, and I like them a lot. As with some of the others, I don't prefer to buy huge sets of cookware. I have a bunch of stainless All Clad pans that I've purchased over the years, and those will last forever. No nonstick pan will last forever--the coating will start to scratch and break down eventually, no matter which brand it is. Hand washing, avoiding metal utensils, etc will help, but you're still going to have to replace the pans eventually.

Sets can be OK if they happen to coincide with your needs, but that's a big if. I've had All Clad nonstick skillets- they're good pans, but they're odd sizes- 1/4" or so bigger than their nominal size, which can cause problems with lids, and I haven't found them any more durable than average for Teflon. I've tried a few ceramic nonsticks, including some expensive ones- they could be good for some types of cooking, but things stick to them when new and it gets worse pretty quickly.

I'm like Nancy, and not a fan of cookware sets. They are too much one size fits all needs assortments, which seldom actually fit your needs. It's better to choose the material and pan according to what the use will be- as Nancy pointed out. I prefer my skillets be nonstick, because I want fond and I want the freedom to stick it in the oven if needed. I've got exactly one nonstick skillet, for doing scrambled eggs and omelets in. The saucepans are nonstick, because it's never pleasant or easy to scrape out cheese sauce or other sticky things. I'd suggest you buy your pans according to what you need it to do, rather than in a set. One size fits all doesn't work for pantyhose or pots and pans.

I'm not a big fan of sets, because I think different cooking tasks require different weights, material, timing etc.
But even if you DO like the idea of sets and want to settle on one (Greenpan or All-Clad), I think the way to decide is to buy and use one of each for about a month, and see how they hold up, how easy to wash, how you like (or don't like) cooking with them.
Choose the test pieces for tasks you do often...frying an egg, making & heating up soup or curry, whatever.
If you like both, buy more pieces, again choosing by suitability for task or most often used.
If you go for a set, you'll still have one of the other for its specific benefits (non stick for the green pan, heat conductivity for the all clad).
Or pass along to a family member or a friend, if you really don't like the orphan pan.