Sharm: Garbage atphomsere, ****ty food, russians everywhere, but good boat diving and chances of spotting biG things also on the local sites. Normally very good visibility. Since you will spend the most of your time on the diveboat anyway, (08.00-17.00 incl transport bla bla), after a shower and a beer, it will be 19.00 before you go down to naama and eat. By 23.00 you will be sleeping. Thay way you avoid the crappy atmosphere anyway.
Atmosphere: ****
Dives: good to very good
Prices: a bit steep, but mostly due to higher operational costs
Overall: My 3rd choice in Egypt
EL Gouna: Disneyland-feeling. few expensive resorts with reletively high standards. Not alot of hasssle, due to the fact that the hall "town" is closed for outsiders so to speak.
Boat dives all day (08-17) with a wast variety of good divespots that are not as busy as sharm/hurghada. Mostly for older people and couples. (Not a big party-scene, just ome pubs... the irish pub i guess is the most lively)
Atmosphere: Quiet and "up market"
Diving: Very Good.
Prices: As sharm
Overall: My 2nd choice in Egypt.
Dahab: Small, "everybody knows everybody". A bit more affected by religion. Backpackers (none of the big tour operators send people here. Small independent hotels/restaurants (except for one Hilton). More young people, majority from USA, Canada, Australia and tthe UK. Many expats that lives in Cairo comes here for diving/hangigng out. Some Egyptians on holiday, scandinaivians, germans, dutch. Small but good nightlife, 3-5 good restaurants.
Diving: Good Value, shore dives, nice rocks, walls, coral gardens, blue hole. Lacks big marine life though. Big unicorns, trioggers, blue sp. stingrays, crocs... but I have never seen eagle ray, manta ray or shark there. El Gouna and Sharm is better for these things.
Atmosphere: layd back, "backpacker light", young, nice mix of nationalities, hippie.
Diving: Shore, great geologi, normally good viz, but can be windy and create surface currents, bad viz, and exclude some of the sites due to entry/exit.
Prices: Great value.
My favourit is Dahab condiering all factors. But I would easily go to Sharm and El Gouna aswell. Its hard to compare....
But: For you: (only a couple of days, not going back for 2 years....) I would say: Sharm if you want to go mainstream, and Dahab if you like to be alittle "off track"...
Hurgahda and Marsa Alam: I havent been there, but I have never heard anything good about Hurghada.
Have a nice trip wher ever you choose to go