That’s a very asked question (we should stick it a dedicated FAQ’s topic), and yet there’s no definitive answer and it depends on personal preference and playstyle.
- The Warrior favor a defensive style, with it’s highlighted skill Counter-Attack and a growth oriented on Defense and HP, but they can benefit higher damage output with War Cry. Following this path leads you to choose between two “prestige classes”: the Knight (Defense specialist) and the Samurai (All-around Strong fighter).
- The Thief in his case arbor an aggresive style with it’s main skill Stun Kick, which generally allows a free critical attack (hitting with the Red Zone of the attack fan) when it connects, his growth focuses on agility with slighty more strength than a warrior. Again, it leads to choose between two “prestige classes”: the Rogue (Agile and Deadly brawler) and the Ninja (The opposite of the Knight: very high strength and agility, but weak defense, also a Master of Deception).
This may be lenghty but i hope it would guide your choice better. As for your question, many people recommand the thief for faring better against NPC ennemies and is generally easier to win with, but playing well a warrior can do well on solo as it does on PvP.
I almost forgot to mention a very important thing about warriors and thieves, their Attack Fan are radicaly different: a Warrior has better length while a Thief sacrifices length for width, it’s a very important parameter in battles.
You can check more details concerning the game here: http://wiki.duelingblades.com/