Summary: Pros and Cons section pretty much explains everything. i advice you to read my updated ruk30 review first to get an idea about this earphone: http://www.head-fi.org/products/koss-ruk-30k-noise-isolating-in-ear-stereophone-black/reviews/11943
there is not much to talk specifically about kdx200 here.
Pros: Compared to RUK30: Slightly better vocals, A little more refined bass & mids, Bright sounding treble
Cons: Treble attack duration is a little short, Price difference is too much between RUK30, Falls behind of KDX100 in overall sound quality
Excerpt: The KDX family of sound isolating in-ear headphones represents the middle band of Koss’ in-ear line-up, with the KDX 200 Silver ($79.99) in turn standing as the middle model in the KDX family.