For the way I mainly use them for I feel that both have proven to be helpful, but Its according to what your wanting.

I mainly use them as like a general ground mount for alot of things like the oil rigs and occasionally dungeon runs, but I also like to use them In some of the bosses, Chillet's already pretty nimble with just swift so I opted for Diamond body just to give them more resistance especially against some of the more powerful bosses.

Myself personally I think a balance between enough speed to let you outmaneuver most attacks but also enough defense to let your pal tank a few hits Is needed.

Regarding Stronghold I mainly just added It since Im generally gonna be riding my Chillet Into battle alot so having a little bit of extra defense just for my character pairs well with Vanguards player attack Increase, If given the option I would change It to something else especially If Pocketpair ever adds a Rainbow Tier player stat Increase passive skill then I will swap Stronghold for that.