1. For quality, meaning good fit and finish, it's a tie. However, I think the Perazzi MX8 is a much better design (MX12, too). It's much simpler, and the way it's designed, it's much easier to work on and is user-serviceable for many things; in this way, it's like the Glock 17 of the target shotgun world. The Krieghoff is unnecessarily complicated, with circlips on every pin (ugh). It requires special tools to work on that aren't readily available to the public, and parts are expensive...pretty much the opposite of user-serviceable, and service is expensive.
2. Probably about the same, with the following caveat: If anything needs maintenance or breaks, the Krieghoff is going to be much more expensive and will likely require a gunsmith to work on; the Perazzi will often be much cheaper and much can be done by the user.
Perazzi leaf springs get a bad rap (I haven't broken any of mine over the past 6-7 years), but even if one did break, it's a $25-50 part that can be replaced by the user in about 5 minutes; a quick swap and you're back on the line. If that's not quick enough for you, you can carry around a spare trigger and swap it in seconds. If a coil hammer spring ever breaks on a Krieghoff K80/Model 32, you might as well pack your crap, because you're done for the day. If you've got a firing pin problem, same thing; the Perazzi can be fixed in minutes (pull the stock, drift one pin halfway out, swap pins), the Krieghoff is going to require several more steps that most users will probably find too daunting and end up sending to a gunsmith. Swapping out hammer springs on Krieghoffs requires a special tool not commonly available to the public, which means you're going to have to pay a gunsmith (probably shipping it off) for beaucoup bucks and who knows how many weeks.
As a gun wears and the lever moves to the right, on a Perazzi, you typically replace the lever spring and fit a new locking block, something that a user can do on their own in a few hours (about $75-100 for the fixture, and about $80-120 for the new locking block). On a Krieghoff, you've got a couple options. Option A is fitting a new lever spring and an oversized top latch (about $200 at the cheapest for the part alone). If that doesn't work, Option B is welding up and recutting the sides of the barrel monobloc where it mates with the top latch...this also requires rebluing the barrel...BIG bucks.
3. I'd say Krieghoff, mainly because I've never had to rely on Perazzi USA for anything. Krieghoff is very responsive to questions and I've heard they're great for sending guns to for relatively quick service and repair; that said, you're definitely paying for it. I've never had to send a gun to Perazzi USA or order parts from them, but folks at Perazzi in Italy have been fantastic about answering sometimes obscure questions about even out of production Perazzi models within a day or two. There are several gunsmiths out there who do good work on both.
4. I tried both and ended up selling the Krieghoff. I'm pretty happy about that, since I was dreading the cost of even minor routine maintenance work on the Krieghoff. To me, Perazzis just seem to feel more light and easier to move; the Krieghoff felt more ponderous, with weight more foreward toward the muzzle (same barrel length for both, same PFS stocks on both). I think it'd be a lot easier to make my Perazzi feel like a Krieghoff than a Krieghoff feel like a Perazzi (then again, I haven't tried a K-80 with Parcours barrels).