Phil,
I have the 28-75 and it's a wonderful lens that's on my camera the majority of the time. Nice, fast focal range for exactly the type of shooting you want to do. I used this lens at an outdoor amphitheater this year and felt it was a tad short at times and found myself cropping most of my shots. I would guesstimate that I was no more than 30 feet from the stage. I had debated on whether to rent the 50-135 to see how I liked it and in hind sight, I wished I'd gone on and done it. It seemed to me that for the tigher close-ups I wanted, I needed something at 100mm or longer. Good idea, if you can, to borrow or rent lenses to see what you like working with in that situation before commiting to a purchase, especially if it's something you will be doing on a regular basis. The 28-75 may just work for you though with some cropping. If you would like to see some photos using the Tammy at a concert you can check out my
set. They aren't that great cause I just got my K20D and wasn't sure about how I wanted to shoot with it, so I mainly went auto and enjoyed the concert.
Good luck with your search! If I had the money I'd go for the Pentax DA*50-135 though. I always seem to think a lens is too short for the situation I'm shooting. The Tamron 28-75 is my walk-about/candid lens. I'm kind of realizing that I'm not a wide shooter but that's just me.