Originally Posted by
Winthur
...I would recommend Long Swords because it's the most common weapon in the Forgotten Realms and it's the easiest to find a magical one. A staple martial artist weapon in BG1 can be found very early in the game and has a +2 enhancement plus cold damage. Generally, Long Swords are always solid in Baldur's Gate; sometimes they're the best possible choice, and even if they're not (late-game ToB), they're still competitive. Two-handed swords are also fantastic; on your journey to the paladin-only sword Carsomyr in BG2, you get goodies like Spider's Bane (which makes you immune to Hold spells) and Lilarcor (immunity to Charm spells). Bastard swords have a few interesting choices and a very good weapon for a Paladin dual-wielder in BG2, the Purifier.
As for weapon styles, for pure damage output your best option is either dual-wielding or two-handing. Dual-wielding gives you a bonus attack per round; in your second hand you should generally put in an item that gives you a ton of utility (example: Crom Faeyr for many martials is basically a "stat stick", because it grants you permanent 25 Strength; Arbane's Short Sword, an otherwise unassuming weapon, is a great help for martials as it grants immunity to many movement-debilitating spells). Generally, you should have enough points to spare that you can experiment, but if you want to dual wield, you better invest in it early!
If you really want to go sword & board, it's not a bad choice, but AC is not the best way of protecting one-self in Baldur's Gate. It's still nice and thematic, and there's plenty of good shields, but they're not as raw-badass-DPS-output. I certainly wouldn't put any points into the Sword & Shield mastery, as it's simply underwhelming for being too situational. If you go dual-wielding, *** in the weapon style is an utmost priority. Two-handed mastery is pretty solid all around...