It is very much a gut feeling, I went with the Z21 rather than the z21 because I did not want to spend money upgrading later on, and I got it for a very good price. As I recall there was a bigger difference between the two originally. The shop I listed is my regular dealer, if you buy from him regularly, you get a further discount as a regular customer. An example is the new NS RAM DE IV from Roco, I paid £375 post free instead of the full price of £465. I place an order with him every year once I have seen the new release brochures, if I need to cancel an item it is not a problem. He will then e-mail me as he receives them and I either pay over the phone, or occasionally I will pay through paypal credit if something unexpected happens. His after sales service is also very good, and he does not mind keeping the models for a few weeks or a month if it is necessary. This has allowed my collection to grow more than it would if I had to pay the manufacturers price.
Although I have a few Trix and Maerklin Hamo models I have to say that they rarely meet the quality of the other main brands unfortunately, although the metal bodies are nice, but it is a challenge fitting modern decoders to the Hamo models. I also have a Trix delta fitted model that will be a real challenge as well I think. One reason I have so little of these two brands is that it is important to me that coaches are the correct scale length and because there are issues with the buffer height of these brands as well. But you choose what gives you the most pleasure in the end.