I personally like Warrior, but it really depends on what you want out of a "spellsword" build.
If you want to be a heavily armored tank that's still deadly with magic, you go Warrior. If you want to be squishy but absolutely explode things with burst damage, you go Thief. If you just want to be a regular mage that happens to wield a sword on the side, you go Black Mage.
Charmsword is a lot more straightforward than it sounds: Strength, Toughness, Presence. Take skills that scale off charm but still benefit a melee combatant like Scaldic Cadence and Song of Storms. Expert Inspiration also makes you a really good tank that dispenses long-lasting party buffs. Shielding Smite and Blessing last for 2 turns instead of 1, for example. Unbreakable lasts for a whopping 6 turns. It's great.