It's hard to keep a carry gun looking pristine. I think you've got three options:
1) If the gun in question were mine, I'd go the home-remedy cosmetic route. I'd pick up some of the stuff available to touch up black alloy frames, and a good cold blue, like Brownell's OxPho-Blue, and just do my best to fix it. I've re-blued a couple of guns using Oxpho-Blue with stunning results. I've used the black touch-up stuff on a Beretta with good results, too. Some metals aren't happy with cold blue and I don't know how OxyPho-Blue will work with the metal in the S&W slide. Brownells's technical staff may have an answer. I've found, however, that heating the metal with a heat gun or hair dryer seems to help. Wipe it down and repeat. And repeat. It's not as time-consuming as it sounds, but wear latex or rubber gloves, as some of the chemicals you'll be using are not friendly. If you cold-reblue it, you'll have to touch-it-up from time to time, but the extra effort is quick, and your total cost will be, maybe, $40 - $50 (much less, if you can find small bottles) and have enough left to do other guns. The alterntiaves will be 4-8 times that cost. The only reason to go this route is $$ savings, but could be pleased with the gun's appearance. Note: this will work, later, if you select the third option, below. Just use this stuff at the first signs of unusual wear in the future.
2) If you're not a cheapskate (like me), the more durable, harder finishes, like the one WVsig first mentioned/recommended, are probably the best long-term solution. And if you're going to spend $200+ (with no hopes of recovering your cost with a later sale of the gun) why not spend an extra $100+ or so and get one that will outlast the gun? Particularly if they can make it look factory fresh. If resale is unlikely to be an option, this is the way to go.
3) A factory-redone finish won't detract from a sale price, while any of these other finishes sometimes can. The S&W redo will make it look like a factory-new gun -- and that will keep you happy for a good while. Should you later decide to sell the gun, and it's still functioning in top form and looking like new or nearly new, THAT may be the best use of your money.
I'm cheap, so I'd try option 1 first, and if that didn't work, go one of the other routes. If it's not satisfactory, you will likely use the purchased materials later... so it's not a total loss.
