General

Let say you uses two-handed sword, and you are fighting boss that has a dodgeable OHKO attack, and you died because the skill delay was 40 milisecond too slow, using a one-handed sword would have given you the opportunity to react by rushing/dodging out of the way instead of dying. But one can also argue that it was the terrible time to attack at the moment. So using a one-handed sword, you can maybe squeeze in one more attack before dodging, where as if you are using two-handed sword you would have to wait until after the boss attack because if you attack at that time, you would have no time to dodge.

1h sword and shield also looks better imho

I realize with the terminus shield sort of back, it is really good to go 1h sword+shield if you want a higher tier potential for your secondary equip.

^That and you can equip a shield.

1H has a lower multiplier than 2H and will therefore yield lower damage.
Of course, 1H hits faster than 2H and with Monster Park Extreme Green potion, the speed cap can be broken.

Once you are able to consistently hit cap damage with either, 1H will be the best for you to use.