I have a 226 Dark Elite and a PPQ M2 Navy. I shoot them often, so I can tell you my .02 for my experiences and my hands and my shooting style - both for fun and IDPA
Triggers are, obviously, different. One is a DA/SA and the other striker. PPQ wins, and I would go on to say that it is the best striker fired trigger in a pistol out there at the moment. Better than the HK SFP9 too.
Recoil: Aluminum vs Polymer. The PPQ, owing to it's polymer construction, will be a tad more "snappy". The 226 is heavier, so it absorbs the recoil better, and the beaver tail helps (me at least) with a higher grip. But, we're talking about a 9mm here; not a 460 SW. Everything is relative. Sig wins
Follow up shots: The SRT on the Sig is amazing, and makes double taps quick and easier. The PPQ's trigger allows me to make triple taps in the same amount of time though. Maybe that's just me, but I prefer the PPQ, ftw.
Parts: No contest; the Sig wins this easily. More parts, more options, more service. I'm dreading the day my Walther needs servicing......
Longevity: I don't know, but the Sig is in service with armed forces around the world and up until recently with the SEALs as well. It's a very robust platform and will quite likely outlive me. There are stories of barrels being shot smooth in a Sig, but the gun keeps working just fine. Plenty of ranges have Sigs for rent too. The PPQ is a Police pistol first and foremost - it was never designed to pass any kind of military trials, so I would consider it not as robust as a Sig. Sig wins.
Quality: Tie. Both are excellent.
Sights: My Sig has the factory night sites, which are quite effective for the distances and circumstances they were designed to be used in. The PPQ came with plastic sights. Assuming you leave them stock, Sig wins.
Magazines: Sig mags are everywhere. PPQ? Hahahahahaha
Grip: Sigs have more options, and, mine has a beaver tail, which I personally like. Mine came with the E2 grips and the red rosewood Sig grips. But, I swapped them out for set of Hogue rubber grips, which I prefer. The PPQ comes with 3 different back straps but no side panels and no other options. I have the largest backstrap on mine and then put rubberized Talon Grips around that. IMO, Sig wins.