Soundstage - at least to me - is the headphones' ability to recreate the overall size of the recording venue. I.e. music recorded in a concert hall should sound like a concert hall, and music recorded in a small room should sound like a small room. Imaging is the headphones' ability to accurately place individual instruments within the soundstage. Basically, it's the relative positioning of the instruments against each other.
Both are hard to judge since there aren't too many recording with actual, natural dimensional information on them. Most are layered studio recordings that have individual layers with individual spatial properties on them, or sometimes so spatial information at all.
I haven't heard everything, far from it. To my ears the HE90 has the biggest overall soundstage, but its imaging IIRC wasn't that great. It sounded somewhat diffuse and overly airy. The K1000 has the best soundstage width - it's truly remarkable in that respect - and its unique enclosure-free design makes it sound as airy as, well, a pair of speakers suspended right next to your ears, since that's basically what it is. However it doesn't have the best depth and height, or the most accurate imaging, though it's by no means bad at it. The Omega 2 has a fairly small soundstage, but its imaging is extremely accurate, and it has an uncanny ability to resolve foreground from background, and make instruments in the foreground really jump out at times. This, however, has more to do with dynamics than soundstage to my ears.
Would love to hear a 009 or a HD800. Soon enough I suppose. In reasonable-price-land, the HD650 has a perfectly reasonable soundstage, fairly open and with good imaging.