Yeah definitely go for the main hand. Without knowing your current gear, it's hard to say, but I'd guess that the MH is going to be a bigger upgrade. Latro's blade is a great OH. It's easily attainable from normal BM. The s2 arena OH blade is an upgrade, but not likely as big of an upgrade as the MH would be for you.
Unless you have the Spiteblade from Kara (which is not easily attainable), definitely save your points for the MH sword. Even then, I'm pretty sure the MH is still a better upgrade.