I have both. I'm pretty hands off and only use for complicated reservations or applying upgrades. I rarely have specific requests to improve my stay.

I've been underwhelmed with SPG Ambassador, mainly because of the inconsistencies across Ambassadors. Some people always receive welcome amenities, while I never do. Nonetheless, the SPG Ambassadors are very prompt and helpful. They sometimes are able to work directly with hotels to bend the rules.

I only recently received My Hyatt Concierge and had two requests. He wasn't able to deliver on either ask, and it was clear if he even tried to contact the hotel for a solution.