A broadsword with 340 AR one handed will do the exact same amount of damage as a Drakesword +5. So the slope here is that 300 AR = 340 with other straight swords for it's light attacks. The heavy attacks of the Drakesword also have a pretty good slope on them, with 300 AR being the same as about 328 AR with a broadsword.
Two handed, light attacks with the drakesword = the equivalent of 342 AR, and the drakeswords heavy attack does 260 AR worth of damage, with the shockwave having power much higher than achievable by a broadsword. (You're hitting about 412 non-counter) This is all for 16 strength and 10 dex. To build a broadsword up to 340 AR you'd need 31 strength 22 dex, or a similar stat spread. You save 27 points, 12 of which, if fast rolling, are then refunded into endurance for the weapons weight difference.
So if the straight sword is only standing on it's own merits... and not those of Sunlight Blade, Darkmoon Blade, or Crystal Magic Weapon, then the drake sword seems to be the obvious choice for a non-magic enforced build. If you're including magic in your build for the sake of weapon buffs though, I'd suggest another straight sword: perhaps the crystal straight sword? It's the longest one and swings slightly faster.