I think this choice doesn't depend on roleplay, it depends on logic. Imagine this in real life. You are about to be executed by the imperials. Suddenly you have a chance to escape. Will you really follow the guy who belongs to the ones who want to kill you? Ok, Hadvar will not kill you after, but at the moment when you have to chose you cannot know that. The most possible outcome at that moment is that Hadvar will lead you to the butcher after the end of the trouble. So Ralof is the only choice for me.
Agreed. You'd have to come up with some serious fanwank to justify any character, even a die-hard Imperial loyalist, choosing to go with the dude who literally just had your head on a chopping block over the one who is about to set you free.
Even when I want to come down on the Imperial side of the Civil War quest line, I always follow Ralof initially, then have a "change of heart" and seek out the Legion later.
My first time through, I didn't know you could go with Hadvar; I don't think I even remember him after Alduin showed up, I was just focusing on escaping. He just didn't make much of an impression, whereas the first thing that happens when starting the game is waking up and seeing Ralof's face. As you're carted along towards who knows where and he's speaking solemnly in what he believes are his last moments alive, most people develop a sort of bond to him and therefore trust him. I may hate Ulfric for being the selfish, power hungry sociopath I believe he is, and I always pick the Empire, but I like Ralof. As someone else said, no matter what race you are, he saves your life and helps you escape, gets his family in Riverwood to help you, and suggests you fight with him against the Empire. With his Nord sense of honor in battle, in a way that says a lot without him having to say anything. Almost like you're kin. If I'm making any sense. Lmao.