Both are quite fun. I played up to a mid-20s assassin in beta and I have a mid-20s brigand now.
I really haven't fought much with Swashbucklers and only have some ranger experience so I'll speak to Brigands, Assassin and the Troubadour/Dirges primarily.
All of the scout classes are pretty group oriented. It's very difficult to solo fight because the armor for scouts [medium at 20+] is just not great. If the tank drops, you can probably stay alive better than the paper-mache wizardy types, but that's about it. Besides just contributing to the dps amount of the group, they have a few abilities across the classes which are highly useful : pathfinding, HO switching, disarm & escape.
Bards are the most group oriented. They have good attacks, but the debufs & bufs that they receive are highly useful. The Bria ballads are absolutely great in particular for keeping power high in the party. You can really continuously pull when those songs are running.
Brigands and Assassins have very similar and very pronounced high dps.
Assassins have nice debufs like reducing poison resist and several attacks which could end up doing big damage. They tend to attack best from stealth mode and shrouded strike helps greatly to get them in that position. In many cases, the attacks which assassins use are from behind or flanking. This makes it difficult for the assassins to solo tougher mobs without careful usage of a tactic like cheap shot to stun the mob, move around back, and shrouded strike/etc.
On the other hand, Brigands have more attacks to choose from depending on what your angle to the victim is. The biggest attack in the low 20s for brigand is 'Ruse' which does around 500 pts of damage on average at level 25, and it's an attack from the front. They also have attacks from flanking positions or all-angles as well. They also have a few spells which are great for groups, like Smuggle. Hell, it's great for groups just to go afk for a couple minutes in the dungeon while Smuggle is up ;)
I have enjoyed the scout classes tremendously in EQ2, even though I tend towards solo classes in general. There's just a bunch of different options open to the scout classes while in combat making the play of them highly active and fun.