I'm pretty opinionated when it comes to cases, and have given it a lot of thought.
My favorite case are these Neoprene slipcases like this one from CaseCrown: http://www.amazon.co...l/dp/B004QJHNJW
At $12, it is hard to beat them at price. The iPad slides in and out w/o any problem, and I like how you can fold the top flap in to make the iPad even more accessible. It also makes an excellent surface to set my iPad on top of so that the back doesn't get scratched.
I really dislike the smart cover. In fact, I blame the smart cover for the scratch you see in my Avatar (from my iPad 2 taken last year..its a nasty scratch in the top left corner). The problem with the smart cover is that it doesn't provide any real padding, and when you unfold it to use it as a stand the surface that touches the screen is exposed to pick up any dust/sand/dirt on other surfaces. Once a piece of sand or something gets in there, the smart cover is in close contact with the glass while having very little padding and the sand will just fester until a scratch is formed.
I like how the smart cover comes off so easily, but I don't think it does the job at protecting the screen one bit.
Most other cases I have a similar low opinion of. The problem with cases is that they enclose the device in spots and any grime that gets between the case and the iPad will no doubt result in scratches.
If you want to keep your iPad looking new I recommend only using a slip cover and just don't ever drop it (ever). That way all the surfaces of the device can "breath" so any dirt or grim is not trapped in one spot left to fester.
Also, with the additional heat that the new iPad produces, the device needs to breath properly to transfer the heat away from the device properly.