Weinmann costs more than the ResMed? In that case profit margin is unlikely the cause for the preference.
In the US the main brands are ResMed, Phillips Respironis, DeVilbiss and Fisher & Paykel. Perhaps they have had a bad lot of lemon Resmed machines. I have no idea about the data or how to get the data on the Weinmann or if it is even important to you since we just have to experience with the brand. It could very well surpass what we are commonly given here for all I know. I recently had a chance to use a ResMed machine for a short time after using a Respironics machine for nearly 3 years. To be honest, both machines did the job quite well and I found that both would suit my needs just fine. I think most people will do well with just about any of the brands. A few might like one algorithm over another but in general, good therapy is available from all the brands. Sometimes the differences are more in the individual models within the brand and not the brand itself. Maybe one has more comfort features or one offers data or whatever.
I bet you will do just fine with the Weinmann. Probably would do just as well with the ResMed. Probably would even do well with the APAP machine I used which costs (here in the US about 2/3 what we would pay for a Resmed). Less expensive $ wise doesn't always equal inferior product in terms of reliability when comparing a top of the line model within the brand.
From what I saw at the Weinmann website, the APAP is a top of the line machine and probably very popular in other areas of the world. We just have no user experience here to offer you a comparison between the brands.