[Radek]

(1) Athletics - among others, it affects time you can spend under water. This gets even more expressed by the Iron Lung trait. Athletics also determines the damage you get when jumping from heights - but this becomes unessential later in the game. I train Athletics to 12.

(2) Thieves: Barbarians have all thieves' abilities, they can train all thieves' skills, they can use all thieves' equipment. In the other words, Barbarians are thieves. Moreover, barbairans (a) get far more HP, (b) can use any weapon, (c) can join Warrior's Guild right in Valeia and train their fighting skills, can join Thieves Guild later without need of "guild hopping" (d) at (warrior's) guild level 5, they get the Stunning Blow trait.
Thieves can do nothing of this: (a) low number of HP, (b) considerable restrictions in weapons and armor, (c) cannot join any guild in Valeia, must wait for Ishad N'Ha (d) no Stunning Blow or any other special trait, everything that is offered to thieves is also offered to Barbarians.
Decide yourself ...