Two things to consider as well are how experienced are you and how likely are you to do you're own wrenching/modifications.
If you're inexperienced and unlikely to do you're own wrenching then I would say that price, reliability of components and service are more important than the last few % in performance.
I don't own any of those bikes. But the Giant, Trek and Scott have big distributor networks behind them which means that warranty work is usually straight forward, but its never completely painless. I can't comment on Norco.
How straight forward the service is, will come down to the individual bikeshop. No two are the same and some get worse/better over time. If you have your heart set on a particular brand, look up the dealer network and ask around the local bike clubs and on here for the best shops in that network.
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