I find this to be a very, very interesting comparison since I happen to be blessed with one of each and colt_saa makes terrific points. I'd like to expand on them a bit and add a couple as well.
We should first talk about how they are alike and at it's most basic... they both are certainly elite-level Performance Center pistols from what may be argued was the absolute pinnacle of semi-auto handgun construction in the entire history of the Smith & Wesson company, which is a very bold statement but one I believe to be accurate. Both were also quite limited in production and as said, don't appear often in the market. And since we are talking possible VALUE, it makes a lot of sense to point out that while they came from somewhat different production time periods, they actually managed to carry similar (original) MSRP price tags. And no discussion of possible value would be wise to ignore how S&W themselves "valued" the pistols at the time they were offered.
As to how they differ and how we should weigh one against the other? Well, it's going to be hell to decide which way to lean... but here some points to consider for each entrant:
845 Limited
The first run of these was supposed to have been around 500 pistols but Mr. Jinks has said there were less of them... I want to remember that it was around 485 and these were introduced in 1995 with an MSRP of just under $1,500. A second run was shipped in 1998 supposedly numbering 150 units, so we can roughly estimate that less than 650 of these fantastic guns have ever existed. The first run was, in my opinion (and perhaps most anyone...) simply more attractive looking than the later run. Unless I am wildly mistaken, the original Performance Center packaging was little more than a plastic Smith & Wesson gun box. The first run of guns were Lew Horton Exclusives... and I'm not entirely sure how the later '98 run was sold, but I can confirm that my '98 pistol was definitely NOT in the hands of Lew Horton. The 845 is a silky-smooth tack driver with an elite trigger and a fine rear target Bo-Mar style adjustable sight. It's thumb safety is a hammer block only, with an enlarged paddle and it has an enlarged magazine release button. Between the two runs, these guns ran from 1995 to 1998.
PPC-9
These pistols were built and shipped later in the history of the Performance Center and you might argue that the Performance Center had developed a few tricks up their sleeve that showed up a bit later. The PPC-9 has a two-way adjustable trigger (in the same way that a 52 trigger is adjustable, albeit using different methods...) while the 845 has no adjustment that I am aware of. The PPC-9 was also shipped with an extremely cool Aristocrat 3-way preset adjustable rear sight, allowing instant preset changes for the different distances of PPC shooting of the day. The PPC-9 also has a magazine funnel/well attached via the grip pin that the 845 lacks and as it does certainly seem immediately attached to any discussion of value... the PPC-9 was shipped in the more familiar aluminum PC case that was the calling card of the elite Performance Center guns of the day.
I don't know the exact range of production but I believe 2001-2004 is "mostly" correct. The 5" version of this gun carried an "as ordered" price of right around $1,600 which is eerily similar to the MSRP of the 845 but here is where there is a significant difference between the two guns that could arguably REALLY affect the value...
The PPC-9 never really had a "retail" MSRP because it was never advertised or offered and was never shipped to a wholesaler such as Lew Horton. The PPC-9 series was available ONLY when ordered by a law enforcement officer and was supposed to be requested directly from S&W on police organization letterhead. So the price wasn't a suggestion or a starting point... the price was what a badge-carrying officer submitted to S&W if he wanted to own a PPC-9. And anything I have read indicates that there were more PPC-9 pistols built and shipped than the hard numbers we have for the 845... but frankly... it just wasn't possible to get a PPC-9 simply because you wanted one. It was sold to LE only and that gives it an exclusivity that we really don't see in almost -ANY- Smith & Wesson pistol that I can think of.
The bottom line...?
If we are pitting them against each other for "value" (and I often put the word "value" in quotes simply because value is soooooo subjective and is often steered, sometimes wildly steered by outside forces having only little to do with the actual gun itself...)
...it seems to me that the PPC-9 edges out the 845 due to exclusivity and features. However, if arguing for the 845 to grab the torch, you can point to the fact that evidence suggests that less of them were built and they simply were made a handful of years earlier than the PPC-9, which is not irrelevant in a discussion of value.
With all of that said...
As was said above in the post before mine, if we are talking about the ultra-scarce SIX-INCH PPC-9 Long Slide, then forget about it because all the discussion is folly. It's rare, it was difficult to get, cost more, appears hardly ever and it's simply going to take this one down. The 6" Long Slide PPC-9 should carry more "value" than either run of the 845 Limited or the standard 5" PPC-9.
Of course... these are merely my opinions, how I see it. Please take a moment to correct anything I posted in error and add anything, no matter how trivial, that I may have forgotten.
These guns are absolutely amazing, and I hope that anyone who has read this thread gets a chance to handle and maybe shoot these pistols.
