In this case despite it's larger zoom range, it's actually the 55-300mm that rates better among users, I've read when stopped down this lens's resolution becomes very close to that of the professional grade DA* 300mm f/4.0.
The 50-200mm rates rather poorly, the 55-300mm is a newer lens.
Everywhere I read people are recommending the 55-300mm over the 50-200mm (it is more pricey though).
There's a great resource over at PentaxForums.com, with user lens reviews for most of the lenses Pentax has ever produced:
link (You'd be looking under DA zooms up the top).
I would personally go for the 55-300mm if I were in the situation.
And yes, as Phrasikleia mentioned you can get some great bargains on some very high quality glass, I picked up a 50mm f/1.4 K mount for ten bucks the other week (it was a great find but you can get f/1.7 50mms off eBay for under $70).
For a long telephoto zoom however you may find manual focusing rather tedious especially if your subject is moving at all, I reckon go with the 55-300mm looks like a really great lens.