General

Since you're a paladin 2h would out-damage a 1h weapon for each attack speed scenario. You'd have to hit around 40-41m damage in order for a one handed weapon to be more effective. The exact number depends on your attack speed.
If you're a Hero, 1h actually produces more damage output if you have +2 attack speed sources other than booster.

@chaosh: paladin secondaries activate shield mastery, like @eggzington said

@chaosh: I was told that paladins can now benefit from shield mastery when using rosaries. That would mean a shield is no longer necessary for said skill, which allows more freedom when choosing 1h/2h weapons preferences. I.e, a 2h sword + a rosary would achieve the same effect, though slower attack speed and possibly higher damage per hit depending on stats, when compared to a 1h sword + a shield.

Do you plan on capping? If so, go with 1h. If not, go with 2h. If damage isn't a concern, go with whatever feels better for you.
Other than that 1h sword + shield lets you easily job change to hero or back since the secondary is also compatible with them. 1h Sword + shield combo also works for DWs.

2h-swords are larger swords that require both hands to be used effectively, so they do more damage per hit, but are heavy and slugish. In Maplestory, that difference translate into 2h swords having higher attack and a greater weapon multiplier, with the drawback of not being able to equip shields (other secondaries are OK) and being slower (Normal(6) instead of Fast(5)). Basically, your range is bigger with a 2h sword (or axe, or blunt weapon, they all behave exactly the same), but you attack slightly slower.

If you don't have a shield with some amount of watk on it (or prime'd), then 2h swords are better, although as a Paladin you need to equip a shield to benefit from Shield Mastery and the watk from the skill helps to narrow the gap between 1h and 2h. For Heroes, you will need a 10+ watk shield or else 2h swords + the regular secondary (a medalion I think?) are better, so for unfunded (or even the decently funded) Heroes, 2h swords are easily the best option.