Tioman is my favourite and have been several times, though redang when i went had some great beaches. Redang i have only stayed at the Berjaya, and have stayed at the berjaya in tioman as well, but have also stayed in salang.Any of the salang accomm will do, its a small bay and i always used to turn up. If you have dollar then go berjaya on either island. Berjaya redangs beach is out of this world!!
Diving - good to great on both. Mainly the macrolife though. some nice nudi's, frogfish etc.. though there is a small rocky island off salang which is nice to snorkel and i used to see bump headed parrots, turtles. Pulau renggis and Talui near by nice dives and has small sharks. Redang has lots of black tip reef sharks not far from where the boats moor up.
Best time of year is variable as its malaysian weather but wet season is Nov-May. I went in April once and the weather not great, but in August it was great.
Nb tioman is nice to explore and has wildlife on the island including pythons, coral snakes, sea kraits, cobras, antelope, etc...so go explore...if u like that!