An XT rear derailler is the same wherever you get it from- in fact most Shimano rear deraillers will fit as a replacement- They are all the same except quality.

Not the same with clothing.It is one of the things you should try before you buy. Saves you the return postage if it doesn't fit.

Lecture over but To a certain extent- Quality is worth it. I used to be of the opinion that if it fits and it is a good bargain- then get it. Have now found the "Better" quality clothing in Giordana and it does pay. Not only on the fit- The comfort of the liner and hopefully the life of the items. Still look for a bargain though but I always get that from my LBS.