So, through playing through the story you get to pick between the mk6 and mk99 partners. Here are their stats which I will be using to make my decision. They should be right... I hope (I'm new to this. I haven't looked into partners much before).

Six:
Slice/Lightning/Mountain+50
Ice/Rain-25

Battle caps at 130 <-I should probably mention that this is 99 at level 99.
Skill caps at 150
Accelerate caps at 15
Defence caps at 20

mk99:
Pierce+30
Lightning+50
Fire+35
Ice+35
Mountain/Rain-25

Battle caps at 100
Skill caps at 120
Accelerate caps at 15
Defence caps at 50


Shared:
hp+2500
hp+15%
sp+150
sp+15%
sp reduction+15
Guard up (forgot the actual name)+30%
movement spd+10
atk+10%
pow+10

Both partners don't seem to have a favourite weapon to eat. They also both cap at lvl 100 and have the same type weaknesses.

Enjoy.

Lemme add their type advantages/disadvantages for ya:

Weak to Beast,
Strong to Monster,
Weak to Specter,
Strong to Soul.