If you want to find out about Mettler threads here in the UK, contact Dawn Cameron-Dick. She has a website and I'm sure she'll answer any questions you have got.
For ease of stocking a shop and for quality, you can't really do better than the whole range of Superior threads (in my opinion). Also remember, that the Superior thread people offer a superb service of advice, charts, etc. via their website. I don't think any other thread company can compete with that.
It might also be an idea to catch the odd workshop with teachers who have a lot of experience in using a variety of threads and include their expertise in their workshops - Philippa Naylor comes to mind.
Finally: After looking at all sorts of makes of threads and colours, the last three bed quilts I made all ended up being quilted with Superior's King Tut.