Most say the 55-300 is better, but the 50-200 is good enough for me. Size difference I'd call noticeable but not huge; it's the *weight* difference that would give me pause, personally the 55-300 is about twice as heavy as the 50-200 (BTW, the stats above are for the regular versions, not the slightly lighter versions sold in the K-x kit). I'd say if you're not concerned with the difference in weight, for $100, the extra reach and better IQ is well worth it. And if you don't get it with the kit, you'll miss your best chance to ever get it so cheap.
Here's a site with specs on on all Pentax lenses (and a lot more useful info too):
Welcome to Bojidar Dimitrov's Pentax K-Mount Page
Also, I'm merging this thread with the similar one you started in the beginner's forum. In the future, please don't create duplicate threads. The beginner's forum was the better place for it. The lens forum might have been OK too, but since this is a common beginner question, that's the best place for it. And before posting, do consider browsing searching for existing threads - this topic is discussed quite often.