Help me select a lens, please!

[cyoung315]

Thanks to everyone. Here are some details I should have provided concerning my current lenses:
Tamron 17-50 f/2.8 with VC (Vibration Compensation)
Canon 100mm f/2.8 with IS (I got this for macro and it works beautifully)
Sigma 150-500 f/5-6.3 with OS
I want to cover that 50-100mm span with something in the 2.8 range if possible. The Tamron 18-270 is a walk around lens I use primarily on another camera.

I've been thinking about the Canon 24-105 and found it for $880 with the Canon US warranty but just wasn't sure about the IQ of a f/4.0 for that much money.

Another question along the same line, I'm considering returning my Sigma 150-500 and getting the Canon 70-200 with a 1.4 teleconverter to keep the range for wildlife shots. My hesitation there is loss of OS and f stops Thoughts?