Tioman and Bintan are both equally good. It goes down to personal prefernce
If you asked me Tioman might be cheaper. (if not going by plane)
Bintan is more of Singaporean getaway so overthere you can pay in SG dollar
Tioman as a nicer beach and some says better than Redang.
I believe Tioman has better things to see than Bintan if you are there for diving
The only down side about Bintan.
if you are going by bus then ferry. it down to your luck in terms of the tide.
The ferry only departs when the tide is high
So example if your package was suppose to leave in the morning it may delays
Including delay total travel time shouldnt exceed 7 hours.
(Bus from SG >> JB >> Ferry Terminal >> Ferry>> Resort)
Personally I would prefer Tioman but Bintan is just more of trouble-saving
Hope that helps