Regarding being unable to close Spydercos one handed.... Spyderco backlocks are easily closed one-handed. They're designed so that the kick or choil hits your index finger when you operate the backlock. I think backlocks are what people are always referring to when they say Spydercos can't be closed one-handed....except Spyderco backlocks can be closed one-handed and are designed to do so...
Regarding aesthetics, if someone thinks all Spydercos look the same due to the blade hole, hump, and leaf shape on their classic models, one could argue the same is true of Benchmade with their more conventional aesthetics, thumbstuds, drop point saber ground blades, and axis locks. The truth is that both companies offer more variety than just what was mentioned above.
Both companies make fine knives. I think it just comes down to what models you prefer. I personally favor Spyderco, but there's nothing wrong with a good Benchmade. I own knives from both companies and enjoy them all, or I obviously wouldn't own them.