Having owned and used both, both are good. :thumbsup:
If you need affordable long tele up to 300mm and you always want to shoot at 300mm, no need to decide - DA 55-300mm. However because of long focus throw, the AF can hunt, especially with older bodies or in falling light. Tested it on K20D and K-7 and there is definitely a difference in AF response. It is much bigger in size and weight compared with the DA 50-200mm WR.
The 50-200mm WR is really nice and compact and focuses quite fast. Where the regular DA 50-200mm has a tendency to show light falloff at the corners wide open, on my test shots with the new WR version, it is not noticeable. I like this lens because from my experience of owning two previous DA 50-200mm lenses, I'm pretty confident to shoot hand held at 200mm with no issues. With the DA 55-300mm, shooting at 300mm handheld can be a hit or miss affair as any hand shake is magnified and can be obvious. Using a monopod with it at focal lengths above 200mm definitely helps with the image sharpness.
Value wise I think the DA 55-300mm is the better option, but prices locally have gone up a bit lately. Seriously, don't buy just because of WR tag, because one can shoot non-WR lenses in light rain with no issues. WR is nice but overhyped imo. DA 50-200mm WR is nice but local prices for it are... :sticktong