Was on Diamond for Circle of Japan in October & Tasmania in March. Just got off Royal for British Isles in June.
Royal is too large for me / Diamond doesn't have International Cafe.
Several plusses for both:
1. Laundry done within 24 hrs & free for Elite
2. Can trade little bottles of alcohol for more water, abottle of wine, or coffee cards and got way more than I could use
3. Art gallery staff pushier than other ships
4. Love the International Cafe staff but there's only food on Diamond. Royal has a FULL cafe
5. Fast disembarkation for tenders
6. 6pm dining on Diamond but not on Royal
7. Beautiful views from Skywalker Lounge on Diamond but a closed in room for Elite bar on Royal
8. Shows were different but good on both
9. More recognizable brands on Royal (Chicos, Tommy Bahama, ect) so I'd wear that stuff afterwards
10. Shore excursion desk has a 30-45 minute 8ntro to ports ON TV so I watched it when I wanted. DEFINITE ADVANTAGE