I love and own both, but Spyderco is my favorite.
If I could only have one knife between the two and I couldn't see what it is before I got it, I'd probably choose a Benchmade. There are almost no benchmade designs I don't like. And my experience with Benchmade's fit and finish is somewhat better than Spyderco's (never had a flawed Benchmade). But I don't have to just have one knife.
That's why I'm a Spyderco guy. I really feel like the knives Benchmade makes are too similar...they're all great, but once I got one or two Benchmades, I had all the Benchmades from the lineup that I really wanted...
But Spyderco has vastly different designs all the time--I prefer the very spydery knives, like the dodo or the para, as opposed to their more conventional lineup, like the delica (where I'd rather have a minigrip).
Also, Spyderco ergonomics are unmatched.
Plus, call me crazy, I just like how they look more.