I have noticed a lot of people prefer a 3 open socket 30% faster r/w diadem over a tiara. As far as I can tell the only difference is the extra ~ 10 defense it might get, are there any other differences?. Does it look different when you wear it?. Any help will be greatly appreciated =).
The only differences that I'm aware of are 1) Defense as you mentioned, 2) level requirement (52 for Tiara; 64 for Diadem), 3) Durability (25 Tiara; 20 Diadem), 4) Magic level (13 Tiara, 18 Diadem) and Item Quality level (70 Tiara; 85 Diadem). Magic and Item Quality come into play for rare items.
If someone is searching for one of those "top" items, he/she probably needs it for PvP purposes, where every little thing counts, so that 10 defense in theory could be a differece between lossing and winning. Also when you pay so much, you want the best you can get.