Personally, I believe that if radio/otf playlist aren't important to you, you should go for the Carbon. I like the Carbon for its shape (wedge, which makes it extremely pocketable), weight (light), style (purely personal preference), and battery life (blows the rest out of the water). The Micro does have replaceable batteries which could theoretically create a very long battery life, but when looking for a very portable player, I chose something that was more built-in. I haven't timed the battery life, but I'm more than pleased with it. Battery life is almost not an issue for me anymore. With my iPod, I was always a bit concerned about how much life was left. With the Carbon, it's not that big of a deal. As far as build quality--it's pretty solid. Not perfect, of course. I like the plasticky-whatever-it-is material used to construct the front of the Carbon because it's pretty scratch resistant. While in theory the metal back is very scratchable, I just left on the cling plastic that came with the Carbon and have had no scratching problems. It still looks pretty good.
I actually did get a Best Buy Warranty. Previously, I had owned an iPod, and it died on me after a little over a year. I'm not familiar with the warranty life, but I'm pretty sure that they max out after a year. Although Best Buy isn't my preferred means of replacement, I'm going to be safe rather than sorry this time.