Personally, I'll take a border war over a long-range war any day of the week. Sending out an army on a 3 week campaign to the other side of the island can be fun, but not if it's the only thing you do.
An enemy at the gate is more interesting than an enemy a week away.
However, if your enemy is a week away, why would you attack your ally next door? Odds are that the enemy probably won't even offer you lasting friendship, so you'd just be making enemies out of everyone.
Closer wars are *generally* preferable. Not in all cases, however. I don't see much of a point in punishing those stuck with less than optimal warfare opportunities. There will be unrest amongst the nobles anyways, I've seen it years ago in many places. People get fed up fast of a boring war, against their ruler mostly and against their military leaders.
Honestly, if all you do is march for a week and then stare at an enemy and return, you probably suck. As a military leader, I know that my army's activity levels depend on how much fun the nobles in it are having with the war, so I always do my best to make what we do as interesting as possible for them regardless of how close or far the enemy is.
