All the companies have their strengths and weaknesses. Here's my thoughts.
Dragon has my favorite MD
-11 mould, hands down. Also, their wings are more detailed because they're made of plastic, so it's easier to put fine details on them. Dragon's winglets are also better than the all-metal brands because of the plastic wings again. Jet-X uses the same moulds as Dragon Wings, and they produce releases in smaller quantities of more specialized airlines.
Gemini is my favorite all around brand. Some of their moulds are superb, some not so great, but overall, they are very good. They use all metal, so they have a good weight, which I like, but it does lead to some funky looking winglets sometimes. Phoenix usually used the Gemini moulds, just in smaller quantities.
Herpa is very hit or miss in the 400 scale. Their 764 mould is my favorite, and their Concorde is also very good. Some of their models have plastic wings, others are metal.
Aeroclassics has the best quality, in my opinion. They focus on classic airliners, as the name implies, and a lot of their releases don't appeal to everyone. If you like 732's, their mould is my favorite, and they have recently released a ton of Air Canada/Canadian 732's, which may interest you.
As for your list, Both Gemini and Dragon have a Westjet 737-700, Gemini's has winglets and Dragon's doesn't (Dragon did release a 732, but it's horrible, and I don't want to talk about it). I think Dragon is the only company that has released the Dreamliner scheme on all the current Boeing planes. The other companies have issued a couple models with the Dreamliner scheme, but only on a couple planes. Both Dragon and Gemini have a FedEx MD
-11. I have the Gemini one and am very happy, but I still think that the Dragon MD
-11 mould is better. Gemini is the only company with a UPS MD
-11, and they have both the old and new schemes. As for the 320 or 737, all the companies make good models of those (not counting the Dragon 732 mould), so you can't really go wrong with any company. As mentioned above, the winglets do look better on the Dragon/Jet-X mould. And finally, for Air Canada, I think that Gemini matches the new colors best. All companies, I think, have Air Canada models, so you should be able to find anything you're looking for there.
If you want to look at close up pics, I would recommend visiting www.400scalehangar.net
, they have a very extensive photo gallery of almost every 400 scale model. Or you can visit www.wings900.com
, which has a very extensive searchable database of models in all scales, but with fewer pics.
Semper ubi sub ubi.