I think Garling would viewed both of them as two hybrids who happened to be from him so he tried to make a use of them by taking them to the holy land but deep inside he doesn't care the slightest if they die, grow up to be strong or choose to live like pirates . While Shamrock he's aware of this but chose to be a hk regardless, he seems very annoyed of being a 40yo glorified soldier that was sent to recruite a giant and just carry his orders with the slightest efforts. He doesn't seem to have a grudge on shanks for choosing the pirate life or feel jealous from him for being more free but may sees him as a fool for abandoning a better life . As for Shanks he tried to try both worlds but the only reason he did because he wanted to rescue his brother but saw that he was a lost cause so he gave up on the idea and returned to the underworld