I restarted my Thalmor RP after I got in bad terms with them (having their hit squads after me kinda kills the point) I am at Unbound deciding whether I should go with Ralof or Hadvar. The character will probably join the Legion once he proceeds long enough in the main quest to finally defect but now at the start he is still loyal to the Thalmor, and I'd like to take some revenge on the captain woman who ordered my execution.

So, if I side with Ralof how will Hadvar be affected? I assume he will live, but how will he react to my character later joining the Legion?

Good question. I haven't tried it. Typically I stuck with the side I wanted to join.

I followed Hadvar and then joined the Stormcloaks. It didn't affect Ralof's disposition towards my character so I assume the same is true of Hadvar.

This time I followed Hadvar then joined the Stormcloaks. It really doesn't matter who you follow in the beginning. The Thalmor only care one thing about the civil war -- that neither side wins it, and from the dossiers I've read they seem especially opposed to a Stormcloak victory. There's something in Skyrim they want, and a Stormcloak victory makes that exceptionally difficult to get.

I always follow Ralof. The Imperials have the better light armor, and are the only ones with heavy armor. It's more of an issue with the heavy armor, really, since no one has heavy armor when follow Hadvar, but I haven't always decided what I want to do with a character when I get to that point.