Thanks Solly - and so here I am!
Diving wise - the reefs in Sharm are, in general, much nicer than in Dahab, but as for the actual diving experience - depends what you're looking for. Sharm is more spectacular, but it's much busier, which means large groups and in the peak of high season, even Shark and Yolanda can be a thoroughly unpleasant experience when the current is pumping and there are thirty other dive groups in the water.
On the other hand, Dahab is a much nicer place. Chilled, relaxed, and with open access to the sea; unlike Sharm, where the beaches are "owned" by hotels. If you can do both, go for it, but if it's just about the diving then I have to go for Sharm ... and I'm not biased just because I work here!
High Season in the Red Sea is from mid-september to mid-october, because conditions are pretty much perfect. The water is warm enough you can dive in a pair of shorts and a rash vest, the air temperature is mid-30s during the day, not 45 like it is at the moment, but my advice would be to come in late October. It's still busy but not crazy busy and after that then business drops dramatically in the run-up to christmas, peaks again over the holidays and then most of the staff who live permanently in Sharm go on holiday in January and February, because it's so smegging cold here! Really!
Some mention was made earlier of tight boat schedules and this is true to a point. Dahab does not really run to a timetable, everything just sort of happens as and when you're ready, but in Sharm the boats really do have to meet jetty time requirements because guides, boats, captains and their crew can have their licenses to work temporarily suspended if they don't meet them - so - sorry about that!
I hope that helps a bit - I try to be as impartial as possible (doesn't always work!) and the regular Red Sea readers have some excellent advice to give also - I don't really see it from the "outside" any more.
And wherever you decide to go, I hope you enjoy it. I have never dived Sipadan but I did plan a trip there once and so read a lot about the place, and also I've been to some pretty cool dive sites over the years(!) but I don't think you can expect the Red Sea to "top" that, especially if you are looking for big stuff (i.e. sharks), rather, you have to think about it as being on an equal footing, but for different reasons.
Cheers,
C.