I have still to receive my K20 with 18-55 ( i have alrady a 35-80 and a 50 a 1.7 ) but i already think about a telezoom.
Unfortunatly i still haven't found a review of a good copy of the 55-300, the only one i have found is on photozone (
Pentax SMC DA 55-300mm f/4-5.8 ED - Review / Test Report ) but they say the lens had a centering defect ( and if happens to a people that bought one, what he can do??? ) so it's quite pointless.
For the 50-200, i have found out slrgear review, and the result doesn't seem that good for me, especially near 50mm in the corners... (
Pentax Lens: Zooms - Pentax 50-200mm f/4-5.6 ED SMC P-DA (Tested) - SLRgear.com! and also confirmed by
Pentax SMC-DA 50-200mm f/4-5.6 ED - Review / Test Report ).
I would shot far and quite big subjects ( especially trains ), let's say not nearer than 20 meters, and i would care about good resolution even in borders.
Actually, in my country they sell the 50-200 in the 150-240 € ( the difference is huge! ) range, and the 55-300 in the 290-370 € range.
I have also seen a Sigma 50-200 dc 4-5.6 ( rated quite well in terms of sharpness on photozone with canon at
Sigma AF 55-200mm f/4-5.6 DC - Lab Test Report / Review but for example has a rotating front element ) for about 150€, and a new Sigma 50-200 with internal autofocus motor and internal optical stabilizer ( who cares? i have already it, but the version without os and only hsm is only for nikon... ) for about 290€ ( no review,
Sigma - Lenses ).