I am into portrait and party shots and also small events shot in churches. Although I use a flashgun most of the time, I wanted a lens that will capture enough light for sharp images without the need to use the flash.
The 24-70 f2.8L (i or ii) vs 24-105 f4L opinion battle has been waged on this forum several times in the past year or so. Most recently in the Canon Lenses forum.
24-70 or 24-105 Photography Forum
As I stated in that thread and various other times, I once owned both lenses and sold the 24-70 mark i. For me, the lighter weight of the 24-105 and IS outweighs the only-a-pixel-peeper-would-care the IQ advantage if the 24-70. I've never had a complaint from those I photograph about the IQ from the 24-105. And to me, the added zoom range of the 24-105 is worth its weight in gold. In short, the 24-105 is on my 5Diii perhaps 80% of the time or more.
Like you, I have found that much of my photography is at small church events (I turned down a wedding request just 3 days ago!). Additionally, I try to shoot with no flash as much as possible. But when I need an 'insurance' shot or two, I turn on the flash and get those shots. As the most of the photos in my gallery will attest, the majority of them were taken in quite low light, no flash, mostly with the 5Diii + 24-105...and handheld as well.
bratkinson - Photos Photography Forum
The first shot, with the toddler, was with my 135 f2L. The train shots near the end were taken with my G15, and the low light shots after those with the 24-105 on a 60D when I owned that camera. In particular, the shots with only candle light were taken with the 24-105 at f4, 1/160th, ISO 25,600...handheld. From what I've read, the 6D has a slight high ISO advantage over the 5Diii.
In short, there's nothing 'wrong' with the 24-105. And for me, the extra 35mm of zoom 'reach' comes in handy more often than not while shooting church events.
Edit: Also, you can't go wrong with the Canon 85 f1.8 for portrait work. Except for the easily corrected CA (in Lightroom), the f1.8 is an all around great lens.