It can go both ways. In these battles the winner is decided if the NPCs shoot the same target with their arrows.

http://www.youtube.com/watch?v=1GYEPDYmRlw

We really need to get a video of Hadvar and Ralof fighting.

Hadvar's more annoying, so he'd win. I'd help him. Can't stand Ralof.

Really? In mine it's the exact opposite with random encounters.

Also, why all the Ralof hate? He's a brave Nord warrior with a sense of humor and a surprisingly unbiased viewpoint.

In the opening, Ralof would murder Hadvar. His axes would rip away Hadvar's shield and bite throw his skull. Afterwords when the rejoined their armies...Hadvar would win because he has the armor advantage. If only Ralof kept his dual axes!

I'll be measuring the pros and cons of each:

Hadvar:

Pros: Pretty decent guy I guess, his uncle gives you a ton of valuables and equipment superior to that which you get from Ralof's sister.
Cons: Is a part of the Empire, which only moments ago tried to cut my head off.

Ralof:

Pros: Is a pretty decent guy, somehow I get a better vibe from him than Hadvar, he seems friendlier. Following Ralof will let you loot imperials whose equipment fetches better coin than Stormcloak gear. Does not have an awkward, out of place accent.
Cons: Less usefull equipment given by his sister, slightly more tedious start in Riverwood.

So weighing those against eachother...Ralof wins, by a lot. Mainly because he's not working for someone who just tried to chop off my head. He's also just plain more likeable.

In a fight, I think Hadvar has superior training, but Ralof has a cause and fights with the promise of Sovngarde should he fall (As he is a true nord, not an Imperial one). So all in all, I say Ralof.