For a light linux desktop, how does XFCE fare against MATE?

Personally, I'd say it's XFCE unless you want eye candy.

On your specific criteria:

1. XFCE wins here, but only just barely if you have a decent GPU.
2. They are both rock-solid stable in my experience.
3. XFCE also wins out here in my opinion. They have all the same standard options, but XFCE also lets you tweak a lot of things at a much finer level of granularity than MATE does, and it's often easier to figure out how to do so.
4. XFCE definitely will, MATE probably will (I would be hesitant to run MATE on anything older than a 4xxx series i3, but mostly because the older iGPU's were frankly crap).

As far as some other points:

• If you're already used to older GNOME, MATE will be easier for you to work with.
• XFCE is less resource hungry than MATE in most cases, but that difference is narrowing over time. This applies to the applications to in many cases (I actually still use a couple of XFCE applications on my systems despite having switched to Cinnamon as my primary DE a year and a half ago for this exact reason).
• XFCE takes up significantly less disk space than MATE, but mostly because of what dependencies it pulls in (MATE inherently requires a lot of stuff that XFCE just doesn't need).
• XFCE offers a small handful of things that MATE doesn't, like being able to switch workspaces by moving the pointer off the edge of the screen in the direction you want to switch.
• MATE has slightly better animations, as well as having better visuals in a number of cases.
• XFCE runs just fine, even with compositing, without hardware accelerated rendering. I'm not sure about MATE.

Why limit yourself to just XFCE and MATE? I use Cinnamon (Mint user) but on my laptop I experienced some shortage in memory while I was developing a large application (I only had 8GB on my previous laptop) so I temporary switched to Window Maker. It's old but stable and only uses about 7MB of memory.