BARD?!
Wow that looks awesome. Not what I envisioned at all.
I'm extremely torn on what class I will play. Summoner is at the top of my list, but I really like the look of Paladin and Bard in this game.
If you had to say how useful each of those 3 are for groups, solo, and duo play, which is best to worst and why ?
Every class is important for group and solo play, the game is pretty balanced as of the latest patch.
Summoner is a damage-over-time class with debuffs. You will use mostly debuff and poisoning skills, together with Ruin from FFXIII for more direct damage while your summon attacks. They are monstrous in long battles and it's common to have 5+ status icons on an enemy when they're around. They are the best at soloing because they can simply summon a tanker pet and never take damage.
Paladin is a tanker based on damage mitigation (while Warrior is a tanker based on sheer HP and a bit of leech). Their damage is on the low end for soloing because of the tanking nature, but they won't die anyway.
Bard is a powerfull ranged DPS (because it comes from Archer), but can sacrifice a bit of its damage to sing recorery and weakening songs. It's the only class that can effectively kite enemies while binding them to boot, so they're pretty good at soloing too.
The best advice I can give you is to try all of them and see which you like the most, because you can (and are required to) play as more than one class in the same character.