Katana, shamshir, cutlass, I never was a fan of curved single edged swords. Nothing against them, I'm sure they cut right along with the best of'em, and over the course of history, I doubt anyone could say with any sort of authority that double edged straight blades or single edged curved ones were responsable for more or less lives, battles, wars, etc.
It's more the asthetic for me on this one. I guess it would be best to say I've seen very much fewer single edged curved swords that I liked the look of than double edged straight ones. Ive seen so few actual examples of straight single edged or curved double edged blades that I'm still holding back before forming an opinion as to where my tastes may lie.