Both Jacob and Edward are evenly matched, according to Meyer. Edward has speed and strength, and Jacob is a werewolf, an entity that has the power to kill vampires. Werewolves are protectors, if you remember from the books, and they are the only real evenly-matched enemy of vampires. This tension between the two is a good plot device -- the two want to fight each other and feel intense hatred towards one another, but they refrain from violence for Bella's sake. The rivalry between Edward and Jacob for Bella's love is a strong driving force in the book series. 

It seems like Edward has more to fight for in a battle between both of them.  If we use their respective relationships with Bella as a guide, the relationship between Jacob and Bella seems like a good friend.  She uses him as the shoulder to cry on when Edward leaves at the start of the second book and while he does develop feelings for her, it seems like Bella will look to him as a steady and reliable friend.  While he does have feelings for her, they will always be muted because the relationship between Bella and he will only reach a certain level.  If we examine the relationship between Edward and Bella, we see something different.  This particular bond is one based on transformation and the desire to escape what is into a world of what can be.  For example, when she realizes that she has powerful feelings for him, Bella wishes to be a vampire, leaving the world of being a human behind.  Edward is poised between being a vampire and denying Bella, or allowing her to enter his world, one he considers to be a curse.  (Assuming only in the first book.)  This level of connection or bond seems much more profound, much more powerful and the type of relationship where there is much more at stake.

If we use this as a guide or a paradigm to evaluate the strengths of both Edward and Jacob, we can see that Edward would probably win in a conflict because he is fighting for much more in his relationship with Bella than what Jacob has.  One other advantage that Edward has is that we know he has supernatural powers and extremely strong powerful reflexes for he uses his strength to save Bella.  We have yet to see (at least in the first book) Jacob demonstrate the same qualities.  We know Edward has the physical stamina to be able to mount a successful offensive against Jacob because of his defense of Bella.