I think this is one of those situations where the brancing class system could have been used in full effect resulting in both jobs being awesome choices like what happened with Summoner and Scholar.

In this case we'd have a tank class that branches out into either a tanking job (Samurai) or a lighter armored dps version (Ronin perhaps?)

In real life, Samurais are the fighters that have masters while Ronins were your traditional wandering swordsmen, so that'd explain why samurai could afford expensive thick armor while ronins had to settle for cheaper, lighter gear.