Second hand Nikkor 12-24? Or Tokina 11-16?

I've been wanting a wide angle lens for quite some time now and have the following opportunities to buy them:
Either a brand-new Tokina 11-16, which I have heard is excellent, if not better that the Nikkors. However, I'm afraid that I might get a defective piece (which has happened to a number of people and customer service where I live is not that great). Also, will it work with future cameras the way a 20 year old Nikkor works almost perfectly with today's cameras?
Or a second-hand Nikkor 12-24 f/4. I just got the lense to test out today and so far the pictures have been pretty good. I can get it for about $550. Unfortunately, the outer glass elements are not so clean; the front and rear have dust and minor scratches along with what looks like water droplets, and the grips are pretty used. The person offering it to me is a photographer who must have used the lens alot.
My budget is something under $650.

Difficult choice indeed.
The nikon is an interesting lens. However, what you describe that looks like water droplets may be fungus. If it seems to be on one of the inner elements I would be very, very wary of that lens. Fungus can be slowed but stopping it requires a total disassembly which will be costly. Was that lend used in a humid environment?
The tokina… I've used one once and was very, very impressed by it. It will get you a stop more light which may be useful.
With regards to future proofing. I've bought and sold a lot of Nikon mount lenses over the years from various brands. The only lens I ever encountered was a 1980s 19-35 f3.5 which happened to have the tokina badge on it (but that same lens was sold under quite a few different names). With that lens my body was convinced it would open to f1.1. Surprising at first but I quickly learned to deal with that.
Now with Canon I know quite a few examples where 3rd party lenses had to be updated to continue working.
Bad samples can happen with any brand, even nikon. Best solution would be to buy the ens in a shop that will allow you to test it in the shop or to exchange it after a few days.
So.think about if you want that extra stop, think about what I said about possible fungus.

The 12-24 mm is an excellent lens and at $550 for a lens that costs $1, 225 is a heck of a deal.
Here is an image shot with the 12-24.
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Check with Nikon and see what it would cost to professionally clean the lens. You may find that the used 12-24 mm may be within your budget. Just offer to share the cost of cleaning with the seller.
Tokina makes a fine 12-24 mm lens ($400). The Tokina 11-16 mm f/2.8 is a stop faster and little wider (not a lot though) and for $599 a good buy.
Which ever lens you buy, I'm sure you will be happy shooting landscapes and architectural shots
By the way. Tokina also makes Hoya filters and some of Pentax's lenses. Recently Tokina bought Pentax, so my guess is you can expect some high quality Tokina lenses in more focal lengths than in the past

I guess it will come down to the question if 16 on the long end are long enough for you, or if you occasionally want to use 20 or the full 24mm of the Nikkor. I own the Tokina myself, and i've found that i take 70% of the shots with it at the 11mm end, 20% at the 16mm end, and the remaining 10% in between. So a few more millimetres at the long end would have been nice.
Then again, the tokina is a bit wider, and i think that every millimeter makes a difference there… And it's faster. Actually even with a 35/1.8 at my disposal it is currently my most low light capable lens as i can use it at three times as long shutter speeds at 11mm than the 35/1.8 and it opens to 2.8 itself. The Nikkor would be a full stop less low light capable. So if you intend to use it indoors in hallways and such a lot, then go for the tokina. Otherwise the nikkor is more versatile but visibly less wide.
I wouldn't really worry about the service… Buy your lens on amazon and test it thoroughly when you get it. The amazon service will be excellent no matter where you live.