I'm very late to this thread and the OP might already be on the trip, but wanted to share my experiences and thoughts for anyone else who is considering the same questions.
I've been to the Red Sea many times by liveaboard and have done a couple weeks land based. In the south, I've gone to Brothers/Daedalus/Elphinstone 2X, Fury Shoal/St John/Zabargad/Rocky 1X via liveaboard. In the north, I've done the North and Tiran route via liveaboard 1X and have also done land based out of Sharm El Sheikh for a week. I have also done land based from Hurghada for the better part of a week and have done some day trips out of Port Ghalib/Marsa Alam.
TLDR, the north is spectacular. Some of the dives sites rival that of my favorites in Indonesia (cc
@Dan !) which is my favorite place to dive. The diving in the north is colder - surface conditions and also water temperatures. I have dived these regions all around Sept and October over the span of 4 weeks each time so I have a pretty good comparison of how temps and conditions are in about the same time period. I just got back a week ago from doing north and south again. Temps in the north (around Hurghada, Sharm, Strait of Tiran, Ras Mohammed) was 75-77F. In the south around BDE, it was 78-82. Folks were cold doing repetitive diving in the north due to cooler waters and windy surface conditions, especially if you're on a day boat. Skip land based from everywhere except for Sharm El Sheikh. The best diving is within 1 hour boat rides from port and there are some trips available that are further to Dahab and Thistlegorm though I recommend you hit Thistlegorm via liveaboard and get 3-4 dives on it without the crowds since you're there early/first/last. The house reefs at resorts are excellent and the Sharm local reefs are very nice as well.
My honest opinion is that the diving in the north is much more spectacular from a reef perspective along with the amazing wrecks. Yes, BDE is "iconic" but the color, density, and "wow" factor is in the north. We even had dolphins underwater! The Thistlegorm wreck is also not to be missed; it's incredible. In the south, if you don't see any sharks, I find the diving underwhelming if you are used to the likes of the coral triangle and Philippines/Indonesia and even compared to the north. It is pretty much all wall diving (though there are reefs in St John and Fury Shoal) but I think the life and appeal does not compare to the north. 3 years ago in Sept-Oct we saw so may oceanic white tips on almost all the dives at BDE, especially under the boat/near the surface/during surface intervals. II saw a couple of hammerheads and we saw 2 thresher sharks. It's considered high season for oceanic white tips. On this particular trip, we did not see a single oceanic white tip (!!) during the same time period. However, we did see schooling hammerhead sharks (14 at the same time!) on 4 of our dives at Daedalus. There was a thresher shark as well. Had we not seen any of that, it would have been a big letdown.
In general, the diving conditions (both in the water and on the surface) are much more challenging in the south than they are in the north. It's mostly zodiac diving (start zodiac, end zodiac) in the south especially at BDE. The zodiac rides are not for the faint of heart and they are rough. Negative entries are required so you need to descend quickly and be able to equalize quickly. The dives are deep - we hit 148 feet for the hammerheads. Our average max depth was 96 ft on the BDE route. You need to have your air consumption down because surfacing early on this itinerary is not much of an option in most instances, the entire group has to surface. Expect strong currents (hi, washing machine) with up and down currents as well. I would not look at how ready one is for BDE based on number of dives alone. You also need to be comfortable diving out in the blue (no bottom, no reference) and surfacing and descending without references as well as confident with your ability in strong currents while deep.
The diving in the north is easier. There are currents there as well, and some opportunities to dive in the blue, but in general the diving is not as challenging or deep in order to see the highlights.