I picked the Barbarian only because I like playing Barbarians. Either one can fill the niche of "Tank" or "Meat Shield" as could a Paladin. Both have advantages but in the end they are both meant to walk up to a bad guy and beat the crap out of him in melee combat (unless you play a Fighter with feats geared towards ranged combat).
Fighters are better for people who want options in their combat. Barbarians are for people who want a straightforward character who fights with raw force. That is really the only difference. I agree with some of the other posters... Barbarian/Fighters are the way to go.
Of course the Bear Warrior PrC is a great one for Barbarians (requires Rage). Being able to turn into a Dire Bear when you rage is always a great thing (+20 to Str and doing Dire Bear claw damage plus newly adjusted Str).
If you do it in this order, you're minmaxing. You come up with the rationale *first*... THEN you take first level Barbarian, other levels fighter. If you do it in that order, you're roleplaying.
I am mocking myself. This is exactly what I did. I made myself an intensely curious rustic come to the big city to see the sights and to learn structured fighting from the masters "... before they subjugate my people with their skills." It's fun so far....
Well I'd say fighter because IMC's there's practically no fighters... always rangers/barbarians/psychic warriors/paladins/monks
I haven't seen a fighter in a looooong time...
Take Barbarian, by Crom!!!! Take the Whirling Frenzy option from UA, instead of Rage.
On a related note, I came up with this concept of a clan of dwarven barbarians, many of whom had been recruited and trained as mercenaries. Take a dwarf with 2 levels of Barbarian with Whirling Frenzy, a few levels of Fighter, give 'em weapon focus (dwarven waraxe) and improved initiative, and you get a pretty nasty dice-n-slice machine. How's that for roleplay, CaffeineBoy?