IHG Diamond benefits are far better than Hyatt Globalist, especially outside US. All ICs in Europe and Asia treat Diamond extremely well.
In the past IHG program was meh. A few years ago they made huge improvements for their top elites (Diamond and Royal Ambassador), making it the best program for those who stay at hotels a lot. Besides, IHG hotels foot prints are much larger than Hyatt, hence easier to achieve top elite status. IHG's IC and Regent brands probably have the best lounges among all programs that top elites can enjoy complementarily. BonVoy's RC does not give such benefit to ANY member, you have to book the room that is eligible to gain access.
On top of that, if you are not Globalist, or even received a gift from Globalist as Guest of Honor, the lower tiers get pretty much nothing from Hyatt. Yet IHG even Platinum from CC often are treated nicely, some properties even went above and beyond to provide breakfast benefit. We had that happened at Kimpton Monaco Philadelphia recently - receive $50 breakfast voucher even just lowly Plat.
