Eruditass has some good points and gave a solid advice. Photozone.de provides some independent reviews that are worth to read in details.
Both the 18-55mm and 18-250mm are good lenses with different strengthes and characteristics: Between 55 and 250m, the latter is the best
, but really you cannot compares directly these lenses with so different ranges.
What matters is your need. Do you shoot in the range 18-55mm or in the range 18-250mm? The latter is really an all-round lens. It has some strong lens distortion at 18mm that can be easily fixed with PTLens (a nice, simple and cheap software). The 18-55mm is much cheaper: as a kit lens, you can get it for an extra $100, very good value for your money.
Lastly what camera body do you use? I tried the 18-250mm on the K-x and K-7. On the K-x, the 18-250mm lens feels heavy. Keep this in mind in your final decision.
Hope the additional comments will assist ....