A friend and i both showed up to take pictures for a local outdoor play production about a year back. He with a new Canon 70-200 (? i think that was the range), me with my pentax 50-135. I talked to him later and noticed that he had taken his new zoom off. Why? i asked. He said that the short end was too long for the setting.
The 50-135 is very useful for a lot of cityscapes and i even get a lot of nice landscapes from it. I use the 50 end a lot, 70 a lot and 135 a lot. If you're into wildlife or bigger events, then i could see the longer zoom, but the shorter zoom is perfect for what i do.
The only negative on the 50-135, is the sdm on it is a bit slow, but i seem to get by and preset sometimes, other times its fast enough. SDM reliability, i think pentax has the problem figured out as the rates of sdm problem are trending down on every lens per Adams recent poll. I have sdm on this zoom (3 years no problems), the 300 (3 years no problems), and also a DA17-70 (which has a fast sdm response by the way) which i just bought recently.
From all accounts the Sigma 50-150 is just as good. The old version was smaller like the 50-135, but the new Sigma 50-150 is in the same large housing, apparently, as their 70-200. I have neither of the latter.
Good luck with choosing -its always difficult