To illustrate the GLA vs PLD thing from personal experience recently:

When I solo or Duo with my CON friend, I go GLA. It allows me to equip a wider range of abilities, I have more attack abilities and attack power at my disposal. I can still tank and keep hate off my CON friend, but it's more of a DPS tank than a heal tank.

Last night I formed a light party. It was me, an Archer, a Maurauder, and my friend Conjurer. Because of this set up, I decided it best to play as a PLD tank. I got more healing abilities, more MP, more defense, and more ways to keep hate. It was such that my Conjurer friend didn't have to switch to WHM to keep the party healed, I did all that. She could nuke instead, tossing a cure on me now and then when things got dicey and I used Divine Veil. Worked really well.

I could've gone as a GLA. But I probably wouldn't have been as effective as a tank.

PLD is limited on solo-ability. You can solo, but it's slower because your DPS output is less.

I'm not aware of how other jobs fair in this realm, but as far as GLA/PLD is concerned, GLA is for soloing, and PLD is for party play.