Depends what level you're going for, and what kind of content you want to do. These Sellswords are boss slayers - with either Lightning, Darkmoon, or Dark Blade depending on enemy weaknesses, they hit for incredible amounts at an absurd speed, and absolutely drop bosses while being plenty fast enough to roll easily. However, the build for this is fairly tight, needing 30-40 Dex and 30-40 Faith before it really gets online, not to mention leveled up talismans.
This also depends whether you want a build with ranged or not. This setup can technically run Lightning Spear, Stake, or Arrow, but damage miracles will ask you to run specific rings, whereas the pure melee setup with just a weapon buff and something like Tears of Denial doesn't need any ring that boosts damage or miracle effects to function properly. Heck, I'm running it now with Prisoner's Chain, Ring of Favor, Cloranthy Ring, and a free slot for whatever I feel like (usually Lingering, but some parts of the game are easier to rush with a Slumbering Dragoncrest to avoid fights). Although with 40 Dex, a bow is doable for ranged, albeit at lower damage than what Lightning would do.
Also consider that they're a bit of a predictable PvP weapon. Tremendous damage, yes, but short range and susceptibility to parries make this a weapon that is very hit or miss based on whether your opponent knows how to zone or anticipate. Not to mention the Sellswords have no poise frames that I know of; in short, if your opponent has heavy armor and a big weapon, they can stagger you while being hard to stagger themselves.