It depends. Some properties relegate Golds to the lounge, while Diamonds are allowed to take breakfast in the lounge and/or restaurant. As to the quality differential it really depends on the hotel. When I was at the Hilton Milano Stucky, the exec lounge brekkie was a joke. Might as well been a Hampton Inn that diced up the prefolded omelettes into scrambled eggs. The restaurant on the other hand was an awesome spread. Other hotels the difference between the lounge and restaurant is limited. One hotel I actually prefer the lounge brekkie is the Hilton Tokyo because they have free pour mimosas up there, and an egg chef. Plus it's way less crowded than the restaurant.