I regularly use both lenses and each has strengths/weaknesses-though the decision to use each is usually based on what I am hoping to achieve at a particular shoot. No two photographers will ever agree on which is 'better', but like any tool, each has its' role.
For an indoor sports/low light shooter, lighting/distance to target play huge role. Angle/background/bokeh also factor in my decision. Shutter speed, need for IS might also prompt me to grab the zoom and in some venues, the situation may require discretion in size of lens, so the 135 may be more acceptable.
For awhile I didn't have the prime and was happy/successful. After investing/expanding my style and tradecraft with the prime, I am capable of a few more things/styles of shots.
The value of this investment was worth it for me, but only you can answer that in your situation.