I'd guess that since the Simond ones are advertised as being suitable for 'fair weather' and are 'lightweight' and the Quechua ones are stated as being 'warm trousers' and suitable for 'winter walking', you've already answered your question.
Yes, I recently bought the Simond ones, and the quality and tech detail is excellent for the price, but they are cool around the bum. This is good for me as I run warm, but I still intend wearing two pairs of pants!
In reply to Muel:
I feel the cold and got the Quecha ones a few weeks ago. They are very nice and I have already worn them quite a lot for winter walking. However they are not as warm as the crag hopper trousers I currently use for winter climbing which are also pretty reasonably priced. So not sure i will use them for climbing in the colder months