The 135mm is too long a focal length on your 1.6 crop sensor David 70-100 is the usual Ideal range for a full frame and thus even a 50mm prime can fiit the bill on the 50D  the 24-105 Zoom is a very fine all round lens and cant imagine you will get better results with a Prime unless you ventured into the area of 85mm 1.2 or the 1.8  which is supeerb value...  Unless you intend to do serious portrait work the lens you have is ideal.

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt