Traditionally
son of gloin sees way more game time than faithful companion. I ask why? As long as you have 4 cards in your hand, you have a str 8 gimli with faithful companion.
son of gloin does have the gandy sig and can be made str 10, BUT he is exhausted after 2 exertions. For staright up skirmishing, cause I know
Gimli, FA is arguably used more than either of these comps, Faithful companion seems to be the better choice. As already mentioned, you only need 4 cards in hand to have a healthy str 8 gimli, you won't have to worry about extra healing, and you can equip him with all the same stuff used by
son of gloin 
. What do you think? Which Gimli do you prefer?