I don't race but appreciate quality. Is there a huge difference between Shimano XT components and SLX components?? Is the difference in functionality or is it one of those weight saving race deals where the price is more in weight saving materials? I'm after durability much more than weight savings. Thanks .

Not just weight but, other things like fine tuning, machining tolerances and quality assurance.

I run XT/slx. works GREAT. Its SLX shifters/front der and XT rear. The XT has given me no issues yet so I havent had to adjust it but once after 500 miles. if it does break then Im gonna replace it with another. The slx shifters are far more than good enough for me. Havent used anything better though. Id say so slx if your not going to race. The "shadow" design is what to look for. It appears shadow gets better reviews than XT too.