I'm a Giant NRS owner, and I'm looking to switch to a Scott Spark, mainly weight saving purposes. Will the ride quality suffer, or improve? Or how is the Scott Spark ride quality?

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Scott Spark LTD 8.40kg

Giant NRS (Retired)
http://weightweenies.starbike.com/forum/viewtopic.php?f=1&t=104531

It's like comparing a bar stool to a couch.
NRS is comfy and goes well. Steady steering and handling is good. Not as quick a bike as the spark for most situations (I'd prefer the NRS on a long day adventuring over tough terrain though).
Spark is a race bike. Firm, fast great handling when really pushing it. Struggles a bit over the really nasty stuff (not what it's really designed for). Flat and smooth?Spark wins. Hilly and smoothish? Spark wins.
Periodically called to do a bit of "all mountain" duty? Keep the NRS.

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Used to race....got too wrinkly and old Updated: Racing again! Thought this was unlikely! Eventually, I may even have a decent race!

A modern spark? I'm not familiar with all of the iterations of the NRS bikes, but a lot of current frame designs might impress you

You're in for a treat, then. Full suspension bikes have come a long way even in the last few years, at least from a strictly ride-quality standpoint. If you run enough sag, you shouldn't suffer in the comfort department either, but you'll get a bike that pedals and steers better.