There is sooooo much discussion we could have about revolvers, but I'm trying to stick somewhat close to the OP's actual questions.
Why a S&W in service/combat/duty size (basically, a S&W that is not in pocket or deep cover size, and isn't a Bear defense gun)
Why a S&W, even pre-1990, over others?
Well, lots of reasons but if we are talking you, now, with S&W versus the other options, it may simply default to S&W based on some of the other options.
The Colt revolvers that can hang in this arena of duty fall in two classes for your use: OLD and either frail or needing work, which is a money pit and/or damn difficult to do. The other class is "GOOD GOD, look at the price!" The prancing horse all on it's own has some aura that other firearms cannot claim. That beautiful logo brings value and high price to every firearm on which it is engraved. I also believe it is gorgeous, so I definitely look at them and I look at pictures of them and then I spend my money on guns I would much rather own that aren't Colt.
Ruger brings a damn good revolver to the table but as has been stated, the double action trigger stroke typically falls short of the feel of the S&W and the tuneability of the S&W. The GP-100 basically runs more L-frame sized (think S&W Model 686) which makes it chunky and thicker than a K-frame, not nearly as svelte as your Model 64. Ruger also does the SP-101, which is somewhere between J-frame and K-frame size. I have never in my life met an SP-101 with a double action trigger that I liked, but opinions vary. A real (and nearly forgotten) gem here is the long-discontinued Ruger Security Six series. (also the Service Six and Speed Six) These were the fore runner to the GP-100 series and were discontinued when the GP-100 hit the market.
These a fantastic service sized revolvers and do compete well against your Model 64. A shame that we didn't have this conversation 5 years ago, they were nearly giving old Security Six guns away. These days the prices have inched upward and they now carry prices very similar to formerly issued S&W K-frames. They are terrific guns if you find one you like with a price you can deal with. Again, the DA trigger stroke doesn't match the S&W in my opinion (and the opinion of the lion's share of wheelgunners) but the guns are rugged and work well. Also, if I have to find their best quality -- I'd say their timing is more solid than S&W, isn't as prone to wear or issues as the S&W is. Not that the S&W timing is a huge problem, but saying that if you dug up 500 examples of each and running the full spectrum of condition, in that sample size you would find a slew of S&W revolvers where the timing is slightly off (or worse) but good luck finding a Security Six (or GP-100) where the timing is bad, because you just rarely ever see it.
To expand on Evan's great post with a slightly more modern look, we need to go back to around 1980 to compare a couple. 1980 was the debut of the L-frame S&W, with the 586, 686, 581 and 681 revolvers. Basically, they beefed up the K-frame and also made the cylinder larger, and they added a full underlug to the barrel. These guns are irrationally popular -- okay, bad word choice, it is entirely rational. They are hugely popular, but if you pick one up with a 4-inch barrel like your 64 has, you may think "WHOA, too much beef here." Especially if you have grown used to the heavy barrel 64.
The L-frame came out to "address the weaknesses associated with full .357 Magnum in a K-frame." <--- this is a monumental discussion that deserves it's own thread.
In any case, the L-frame has been a raging success and Ruger answered that success when they debuted the GP-100, I wanna guess it was about 1985 or 1987 or thereabouts. The GP-100 is more similar in overall size to the L-frames. Ruger fans seem to love to point out that the frame is larger, fatter, beefier and appears "stronger" than the L-frame, and they were spurred on by many Ruger print ads back at the debut of the GP-100 that pointed this out visually. Smith & Wesson fans counter with the fact that the L-frame (well, ALL S&W revolvers...) are forged steel while the Ruger is investment cast. The Ruger needs to be thicker and beefier because it isn't forged steel. Either way, that argument ends up being codpiece waving, since both revolvers will take a beating beyond what most shooters have the time, ammo, money and wrists to put through them.
Back to your opening post. Why a K-frame over others for what you are doing right now? Well, a USED, formerly issued K-38 (10, 15, 64, 67) is just easily the MOST gun for the money and it has millions of examples to prove it's success. If you are buying new, I'm looking hard at a GP-100 because the S&W they ship today are shoddy, IMO. QC has seemingly been outsourced -- directly to the end buyer. Their customer service at S&W continues to enjoy a solid reputation, and WOW, they sure get to exercise it. A fly on the wall might suggest that the guns coming in for warranty work outnumber the new guns coming off the production line. But alas, that too is likely it's own thread.
What else is new in revolvers on the market right now? Well, Dan Wesson is back, under CZ-USA production, and these have stuck totally to the original DW design, but they are better built and finished. The price tag is HEAVY, a new one runs some $1,300+ MSRP. Colt has reintroduced the Cobra, and so far, no other double action revolver. That is J-frame sized or smaller, so it doesn't fit the criteria for this discussion. Taurus makes a crapload of products... and if you have a dozen given to you, I am certain that you could find a good one or piece together a good one. Life is too short to bet your life on one unless/until you can beat the crap out of it for a few weeks and see how it responds. Under Taurus, file Rossi and Charter also. The Rock Island Colt copy and EAA Windicator, a German made budget gun, these aren't serious contenders in this role.
Korth is not only still making top-drawer revolvers, they now have a US importer deal. Unless you have $3,000 and up, we can save this for another thread. Manhurin has also began making the MR-73 again, with a price that rivals the Korth. Kimber has a new revolver out, it is also on the smaller side. I played with one a couple weeks ago, it seems solid, heavy DA and pricey. Not exactly 4-inch K-frame sized.
Chiappa has the Rhino. A big departure from what we know and expect from a revolver. The quality is suspect and the price is double what you spent on your Model 64.
Options for a contender to your Model 64?
Security Six (adjustable sight), Service Six (fixed sight) Speed Six (round butt grip frame) if you find a decent price. Maybe some Colt revolvers that I am not qualified to suggest by model, assuming you bump in to a widow that is selling them according to the original price tag on the box, not adjusted for inflation or collectibility. Otherwise, nothing from Colt.
I like to swap brass... and I'm looking for .32 H&R Mag, .327 Fed Mag, .380 Auto and 10mm. If you have some and would like to swap for something else, send me a note!