As of now I have to say the long espada is probably one of the more effective swashbuckling weps. It has balanced stab and swing damage, meaning it performs admirably at both (and because of this, has a pretty high stab damage). It is also on the longer side, which means that it is pretty good for all swing directions so long as you are careful with the overheads. Many shorter swords aren't as good at doing some swings since anyone with a longer 2hander or polearm will outreach you in all directions. The short spammy 1handers are great of course if you are going for agility. With low - medium athletics though, you want a good length.
Personally on my shielder I use the spathion, since I pick all my loadouts simply on looks alone, and then force myself to become proficient with that weapon. I like to pretend that it is a spatha rather than a spathion... that would make it a roman cavalry sword, and roman swords are incredibly sexy considering their simplicity, imo. The spathion is very similar to the long espada, simply slower and has more cut damage, meaning it is better for swings rather than only stabbing (which you can very easily pull off with the super fast long espada). I find it very easy to get stuck playing as a 1handed hoplite with the longer 1handed stabby swords, so unless you want to be really predictable, make sure you switch it up. The spathion helps you remember to swing since it hits pretty hard. The long espada however... well... pretty much everyone using one I have met since the recent patch is a 1handed hoplite.
The long arming sword seems like a decent middle ground between the long espada and spathion... it is simply a faster but weaker spathion. Looks like a decent choice if you couldn't decide between the 2.
Also, if you are planning on high agility, and maxing weapon master and athletics, then I would suggest a shorter, faster 1handed sword, like the liuyedao or niuweidao. With 8+ WM you can spam those things faster than people can block, no joke. Of course you will be missing a stab, and as phew pointed out you will be more prone to block stuns, but with high agility you can simply dodge their swings with practice. The shorter nordic swords aren't a bad option either.
tl;dr: experiment and find what works best for you. Or, find something you find sexy and just get used to it.