I would say Hexblade 1/Swords X is probably better overall, because Bard is overall one of the best classes in the game, and Hexblade 1 is probably the best dip in the game. (Second best is Hexblade 2.)
But, Hexblade single class does have some advantages. The relative utility of single class hexblade depends on how much you value them.
1) Faster spell and feat progression. Being a spell level above at 3, 5, 7, 9, etc. isn't a huge deal, but it isn't nothing. Likewise, being one level ahead for feats doesn't hurt, especially if your game progresses relatively slowly.
2) Short rest versus long rest. If your game features lots of short rests relative to long rests, warlock becomes more attractive, even with Swords bards getting more BI dice. I'd rather have two top level spells over 5 BI dice.
3) High level warlock spells and invocations. Eldritch Smite, Improved Pact Weapon, Lifedrinker, Tomb of Levistus are all warlock only, and all are very good boosts to offense a Sword bard isn't going to get. Likewise, Shadow of Moil(!), Hunger of Hadar, Banishment, Hunger of Hadar, and Cone of Cold are all spells the Warlock has access to that the Sword bard would have to spend his two precious magical secrets on, and not until level 11.
My next character is going to be a level 8 Hexblade, and I thought long and hard about going Hexblade 1/Swords 7 instead. But being able to pull off the Shadow of Moil/Elven Accuracy/Greatweapon Master combo right at level 8 was simply too tempting.