The question 1:200, 1:400 or 1:500 is sometimes answered in a temper so hot that one would think it was rather "Jewish, Christian or Muslim?" (PLEASE don't start a religious discussion, thank you very much).
1:200 is clearly the scale where you get the most details on the models. There is a variety of brands to buy from, all pretty expensive. And you'll also have to consider the space you need for displaying your models (or even storing them in a closet).
1:400 is pretty nicely detailed, there is also a nice variety of manufacturers you can buy from. However, the models are still expensive.
1:500 models are not that detailed like their bigger "cousins". But they are the cheapest scale to collect and also the one where you can put your models on display very easy. There are a couple of manufacturers, by far the biggest of them being Herpa (Germany), producing all kinds of liveries, with the exception of Far-East airlines, which are produced by Hogan. There are also smaller brands like Aero500, Sky500, Starjets (Schuco) etc., who sometimes produce better quality than scale-leader Herpa - but you must be aware that those models are usually more expensive. There is also a decent variety of airport buildings and accessories for the 1:500 scale, so building your own airport to put your models on display should be pretty easy.
When I started collecting, I saw that tiny little DHC-8 model in 1:500 I found it so nice I had to buy it. It was 1:500 (and that's the one you can collect in Europe fairly easy without having to depend on Internet sources) - so my slowly growing collection is 1:500.
It wouldn't be a bad idea to consider first which airlines or types of aircraft you are interested in, prior to collecting.
So, to sum it up, IMHO there are some factors you could consider:
- availability of scale in your country (yes, there ARE internet sources, but still...)
- availability of desired airlines/aircraft in a scale
- space needed for display
- simply storing or displaying in a diorama (thus needing buildings, accessories and vehicles)
Just my two cents. But the most important thing collecting model aircraft: HAVE FUN DOING IT!