Dragons are the most popular in the 1:400 scale. Gemini Jets are also very popular.
Dragons: metal fuselages, plastic wings, and tails. Wings are more detailed, very beautifully assembled. Wide range of products, from the regular releases to corporate releases and other special releases. Many go up in value after a while, so they are good investments too. Always check inside the box before purchasing, because many come damaged from the factory. Check for tail angles, if all engines and wheels are attached properly. Quality seems to have improved recently, but there used to be many that would be delivered with problems. All in all, Dragons are my favourites though.
Gemini Jets: all metal. Much fewer problems compared to Dragon models. Just recently, the moulds have changed, at least on the 747 so the ugly assembly of the horizontal stabalizer especially has been greatly improved. Some extra details on Gemini's that are missing from dragons, including engine markings and aircraft lights. You can now put a Gemini beside a Dragon, and only collectors will recognize the differences, since they are so well put together.
Whatever you do, find a good supplier who you are comfortable with and you can get competitive prices from. Good luck with your new (expensive) hobby!
"it's kind of like an Airbus, it's an engineering marvel, but there's no sense of passion" -- J. Clarkson re: Coxster