I have a lvl. 13 Warrior and I want to change his class SOON, but I don't know what to pick. A Barbarian or a Samuri? I already have a ninja that has all the theif skills and so having the barbarian with theif skills is pretty useless. Other than theiving skills, is there anything exceptional about the barbarian? I want this character to be my most powerful fighter. Would a Samuri be a better choice for this?

IMO, Samurai is by far the better choice. Two words: "Sun Magic". Many of the best offensive spells are in that area. Even if you have a wizard with Sun magic, it never hurts to have another character with it.

The only benefits of Barbarian role are the massive HP increases (which you have to roll and reroll and reroll and reroll every level up to get, grrrr) and the Thieving skills (which all you need is one character).

Technically, if you move someone with Vine magic up quickly enough, you don't even need a Thief, with the 5th level Vine spell "Disarm Trap". Of course, your sorcery must be greater than or equal to the trap level...

Samurai can also learn "Tiger Strike" (more rapid fighting), gain Ancestral Guide (better "to hit" bonuses) and "Deathstrike". Make sure you have him/her learn Doublestrike and then start adding points to 2nd weapon. When you do the First Bushi Dojo quest, the Samurai will give you a +1 Wakizashi which makes an outstanding 2nd weapon.

Have fun!

-Sazerac

You might try to do both.
Barbarians have some nice specials too, apart from the hitpoints: They start with fury which gives them a chance for double damage, and at guild level 5 they may learn stunning blow, with a chance to paralyze a monster after each hit.

...Silence
There is only Silence
(it may take a while yet before you find where this W&W quote comes from)