As a watch collector, if you are going to only have one of our loupes, then by all means that should be the Model 01 6x loupe. It is the one I designed for myself, as a collector, when I could not find it on the market.
The 6x is unique in that it will give you the full view of a movement or of a dial at once - you do not have to move either the watch or your hand, and you can just let your eye roam since the image is equally sharp across the whole viewing field.
The 10x, instead, will allow you to dive into the smallest details, with an optical quality comparable to that of table-top binoculars. In that it is very complementary to the 6x, but I would always recommend to start with the 6x, and then perhaps later add a 10x with light.
The 3x was specifically designed for brands to use in a retail setting, and offers a huge field of view, great for first discovering a watch in a sales context.