Corwin said:
The title says it all. What are the advantages and disadvantages in practical terms of selecting one side over another? I know the Duke or Counts will give you quests, but what else? If I join one side can I still visit towns from the other side; buy and sell there? Will I be automatically attacked etc by scouts or soldiers from the other side? Other than some quests, why shouldn't I stay neutral?
If you declare allegiance to either faction, you can still buy and sell in towns of the other faction. I don't think there's even a penalty, though I could be wrong.
You have a reputation score with every faction in the game: Vaegir, Swadian, river pirates, mountain bandits, Kerghits, Dark Hunters, etc. The lower the score, the likelier members of that faction will attack you on sight.
My main character is around -38 to Swadians right now, and he can count on being attacked by Swadian war parties whenever he crosses paths with them. Swadian foragers usually run from him, and caravans still mostly ignore him.
Interestingly, in the newer versions of M&B one can bribe certain factions for a temporary improvement in one's reputation. Paying off a group of mountain bandits, for example, will raise one's rep from -30 to -1 with all mountain bandits for a day or so.
Declaring allegiance to Vaegir or Swadia allows you to get quests from the lords, and more importantly allows you to prey on the other faction. Attacking caravans, foragers and war parties gives much better experience and loot than attacking the same old pirates and bandits.