*possible spoilers* Does it make a difference, later in the game if I choose one or the other, specifically? I would like to know if this choice affects anything at all, later on. Quests, gear, unique reward items, etc. Thanks.
I'm pretty sure it barely affects things. You still end up on the same storyline after leavin Riverwood. It also gives you different things in Misc.: either Join Imperials or Join Stormcloaks. I think. --- The dragons call me Dovakiin. The Nords call me 'danger-prone.' They also call me a dirty thief.
I want to be completely sure before I choose. I am not planning on doing any quests, until more information hits the wiki/boards. I just plan to explore, and level. So I don't want this to end up being a repeat of Oblivion.
Where I gave the amulet of kings to Jauffre without knowing that I could never get the Emporer's robes and shoes after giving it to him. That was a huge early game/beginning game secret. I was 200 hours in, before I learned.
It just gives you one of both missions, and another excuse to visit the first town. The other one can simply be started by talking to people from either faction. I'm pretty sure it's just a way of introducing the civil war factions. Both doors into the keep lead to the same cell. You could see it as a way to obtain faction armour by joining the opposition. Those armours are far from unique though. --- For future reference: landing in active vulcanoes tends to fry our sensors and melt our hull.
It doesn't really matter... Following the Stormcloak guy gets you out of the tutorial like 2 minutes faster. Following the Imperial soldier lets you grab three swords to start with and you can snatch some iron/steel ingots from his uncle the blacksmith for free and make some good starting armor. You have to steal them if you followed the Stormcloak... But you get a key to his sisters house, where her husband seems to get stuck sometimes. So selling lumber to him early on can be a pain...
Otherwise, just talk to people and you get the option to join either group, regardless of who you follow.