I was happy with the 28-135 on film. I was happy with the 28-135 on my 20D (matched nicely with 10-22). Hated the 28-135 when I went to a 5D. Replaced it with a 24-105 - huge difference.
See what Canon did when they "updated" the 24-105? Produced a 24-70/4 with less range, similar quality and double the price. I say leave the 24-105 alone. Be careful what you ask for when updating the 28-135!
Me, I'd rather have a 17 prime (2.8 preferable, but f4 okay) as the 20/2.8 I had was not great, and there are plenty of reports on the limitations of the 17-40 at 17mm.
If gear matters: 6D, M3, Samyang 14/2.8, Voigtlander 20/3.5, EF 24-105/4, EF 35/2.0IS, EF 50/2.5 macro, EF 85/1.8, EF 200/2.8II, EF 400/5.6, EF-M 11-22/4-5.6, EF-M 18-55/3.5-5.6, EF-M 55-200/4.5-6.3, Ext 1.4x, Lifesize conv, Ext tube EF25, 430EXII, 270EX, Yongnuo 603C