Well, single semi-auto pistol is hella quick, so its a significant advantage. Depends on playstyle, really. Boils back down to the ol' Commando choice - Tactical Reload vs Extended Mags? (I swear, one day, we shouldn't have to choose between these like we used to) Except with Gunslinger, its faster reloads -and- less weight. Granted, Winchester, or LAR for short, isn't meant to be there in the end, but its a super attractive option from Wave 2 until however long it takes to need to drop it to free weight up for other things. I've seen some Gunslingers using it, and I use it to my own discovery and merit. Its not necessarily about just having power early on, but getting money quicker.
Consider this: The LAR does 80 damage per shot, putting it right behind the Desert Eagle, but it is accessible right from Wave 2 at an attractive $200, leaving plenty of money for ammo. Problem is, due to its power, its ammo is overly expensive. $50 per reload - .500 Magnum levels of expensive. Keeping it fed through normal means will easily chew up any profit you make at that range in early waves. But here's the kicker - buying it from the Trader starts it with 60 ammo - $250 worth of shots. Seems normal, right? But due to its high ammo cost, its anything but normal.
So, the trick is, why buy ammo? Seriously, why? Just buy another LAR. It not only costs just $200, but it sells back for $150, netting you back 75% of your investment instantly. It's that simple. Wave 2, buy a LAR. Blow through it, and fall back to 1858s or whatever you have. Wave 3, sell off the used/empty LAR, and immediately buy a new one. Boom, just basically bought $250 worth of ammo for a mere $50. Quickly adds up during the first half of a match, especially 10 wavers. Allowing you to stock up on those pistols that much faster.