Dave,

Not sure if this weighs in on your purchasing choice or not, but I had the 24-70mm f/2.8 v1. I never really enjoyed the 24-70. I sold it and purchased the 24-105 and couldn't be happier. Not sure if the t-stop difference is anything, but I for one prefer the extra zoom of the lens. I have lots of primes and zooms and this is on my camera more than any other lens. I know you think it would be an upgrade, but look back on your shots with this lens and see what is the majority of mm you have used it with. If it is above the 70mm range, I wouldn't change. I always found the 70mm limiting, especially on a full frame.