Turtling and critical damage potential is why the Estoc is most oft praised.
Higher base damage than its competition when elementalised, not to mention greater range to boot, and has boosted Critical Damage as a result of being in the Rapier class of weapons. Basically, this means that it has enhanced backstab and riposte damage, and like most other critical-damage weapons is especially deadly when paired with the ever-present Hornet Ring.
Otherwise it can readily be used to poke at opponents from behind a shield so you've got a simultaneous offense and defense going on, unlike the Balder Side Sword, though the latter is still an excellent weapon in its own right.
http://i.imgur.com/PELu7.png What are YOU playing?
One more plus for the estoc: it's also very useful for fighting in cramped spaces.
That, its speed, and the ability to turtle with it is why it's been my favorite weapon going all the way back to Demon's Souls. That's not to say it doesn't have its weaknesses, but it's still my favorite.
Both are awesome and in my list of favorites, but time for some Balder Side Sword positives.
It is the longest straightsword in the game (I'm pretty sure), Swings don't take up much stamina so you can chain massive combos, the R2 lunge is ridiculously powerful (and is also good in cramped spaces, the damage scales well with dexterity, and can be enchanted.
When I did a rapier crit build I went with a estoc/ricards then kept my balder sword as my second weapon. So I say use both at the same time. Estoc to start get the backstab then switch to the speed of the balder sword to catch people off guard