Is it worth the extra hundred for the 300 mm?
I think it depends on your need for sharpness vs your desire for small size. Many copies of the 50-200 are a bit soft at 200mm while the 55-300 is typically a bit sharper at 300mm and much sharper at 200mm.
If small, lightweight and compact is your goal with the K-x then I don't think the 50-200 will disappoint. If your goal is the best telephoto images possible on a budget, $100 extra isn't too much to pay for greater sharpness and reach, IMO.
Personally, I could live with the plastic mount and foil stickers instead of plastic badges on any of the DA-L lenses. On the telephoto lenses though, I'd miss both the lens hoods and the quick-shift. Case in point - shooting close-ups of helmeted sports players. AF will virtually always focus on the face guard leaving the eyes soft. With quick-shift, it's easy enough to give focus a touch-up and get the eyes sharp with the face guard soft - ideal. I'm not sure how I'd do that without quick-shift. I probably just wouldn't try those shots any more.
The DA-L 55-300 costs $200 over the one-lens kit while I just got the DA55-300 for $272 shipped. For me, it's worth $72 to get a metal mount, real badges, a lens hood and quick-shift. I expect that I'll probably sell off my DA50-200 soon enough as I don't see why I'd ever use it with the better DA55-300 in my bag.
DA50-200mm vs ZD40-150mm (test shots)
http://forums.dpreview.com/forums/readflat.asp?forum=1036&message=34013768
I hope to have test shots up next week comparing my older DA50-200 to my new copy of the DA55-300.
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Group Captain Mandrake: 'I was tortured by the Japanese, Jack, if you must know; not a pretty story....Strange thing is they make such bloody good cameras.' (
Dr. Strangelove , 1964)