The grand suites fall into the Sky class of the Royal Suites program specific to the Oasis and Quantum class ships. So the suite experiences are just a little different compared to the rest of the fleet. As you noted, is it better or worth it is really subjective.
Probably the biggest perk out of all the items is access to the lounge and Coastal Kitchen. We have done the transatlantic on
DCL in cat T and done a few different cruises in suites on Royal. The Oasis class wins the head to head for the suites lounge. Especially if you are not doing the UDP then having all day access to CK really would sway me. We have access to drinks elsewhere so only grabbed drinks in the lounge in the evening but reports are besides the free drinks during happy hours, the lounge has free beer and house wine the rest of the day (Needs confirmation).
The extra space in room is nice, and the suites seating in certain venues can be really helpful. The free VOOM internet (one device per guest in the stateroom is a nice perk. The suites sun deck on that ship does include a hot tub which we found (on a sister ship) was much less crowded, even though it was smaller. Those a a few things to consider.
As for Key vs. Suites - at boarding Key boards after suites, the internet is included in both. The boarding lunch might be attractive with the key but CK lunch is probably close and if you have the UDP then it really is not a perk. The edge for the key would be the special times for flowrider, etc. That may not be as much of a benefit depending on your group’s interest in those things. The other fact is a TA, with so many sea days and age mix, that may ease some of the activities demand. Not sure if the carry on bag drop is a huge perk for the key but that is another difference.