So the three questions you have to ask yourself are:
1. Will I miss the gap between 50mm and 70mm if I get the longer lens?
I don't think that you would miss if very much, if at all (but it's really going to be your call). And having the gap also gives you the excuse to get something like a good 58mm prime to fill it in later on.
2. Will the extra weight and length of a 70-200mm lens be a problem?
The Sigma 50-150mm is 5.3" (135mm) long and weighs 1.7 lbs. (780g), while the Sigma 70-200mm is 7.3" (184mm) long and weighs 3 lbs. (1370g) and the Tamron 70-200mm is 7.6" (194mm) long and weighs 2.5 lbs. (1150g). I doubt that the extra length would be any problem, but it is a significant difference in weight. Personally, I don't think the difference in weight would stop me if I wanted the longer lens, but again, it's your call. (I wanted to add that you should be aware that even the smaller lens weighs significantly more than your Pentax 55-300mm.)
3. Will I miss the 150 to 200mm range if I buy the shorter lens?
That's hard to say. I definitely use my 80-200mm lens at 200mm a significant amount. I don't think it's too long for event photography as long as you have the complementary lens for focal lengths 50mm or less. At this point, I would miss the gap from 150mm to 200mm if I didn't have it. I'm not as certain that it would bother me if I had never had this covered with this lens. My gut says that I would miss it though. You always seem to run into situations where extra reach would be nice even if you've never had the extra reach in the past.