I've been diving for week long trips at both Red Sea Diving Safari (Nakari location, south of Marsa Alam) and Dahab.
Dahab is a small town with hotels and accommodation of all levels (1* up to 5*), Relatively quiet. Shore diving predominates, although there is some boat diving to be found. We stayed at the Meridien hotel when we were there in 2009 = 5*, very sleek and new. Poseidon divers is the operator at Meridien....they also have a main dive school in Dahab town (Meridien is a few miles south of Dahab town). Poseidon ran 3 boat trips a day from their Meridien location, mainly to sites within 15 minutes of the hotel dock. They also ran various dive trips every day to locations up and down the greater Dahab coast (Blue Hole, etc.).
Meridien hotel, Dahab:
Outer wall of the Blue Hole, Dahab:
RSDS (at least at the Nakari location we were at) is VERY quiet. Eco-lodge/dive camp/Robinson Crusoe-in-the-desert type of vibe.....really nothing in sight around the Nakari location except desert and the sea. Accommodation is in simple nubian chalets or tents. No TV, entertainment, fancy palm tree landscaping - just all the diving, snorkeling, desert and quiet time you could ever want (bring a few books). "Wild" camels wandering through the camp at times. Many, many diving options = shore diving from the camp location, zodiac diving to/from camp (or shore dive out/zodiac back, vice versa), daily boat dives to dolphin house reef, shore dives up/down Marsa Alam coast via truck, day trips to other RSDS locations such as Marsa Shagra, where you can do a day trip out to Elphinstone, daily night dives at the camp location. Note that Nakari is a smaller location; Marsa Shargra is their main base....I think it is larger, but still relatively simple.
Red Sea Diving Safari (Nakari):
Shaab Samadai - Marsa Alam:
Overall I found the diving down in Marsa Alam a shade better than Dahab = just a little quieter and slightly more consistent high quality dive sites. That said, we visited RSDS in 2007. There looked to be a lot of building going on in Marsa Alam at that time....I assume this means more development, more people.....
Anyway, I don't think you can go wrong with either of these destinations. We really enjoyed our experiences at both locations. Dahab also has the advantages of being closer to non-diving attractions such as Mt. Sinai, St. Catherine's monastery, quad biking in the Sinai desert....we also did a day trip to Petra in Jordan while we were there (very long day, but fantastic trip).
Hope this helps.
Chris