Honestly, if you don't need a fast lens for DOF then get the 17-70. Outdoors, in good light the picture quality will be comparable assuming you do your part. I don't do flash, and you won't either in a church in Europe I'm pretty sure. How often are you going to do that vs. outdoors? I have a Tamron 18-250 I use for travel/when I'm feeling lazy and for the most part I can't tell the pics apart from my faster glass after processing. Sure, the 18-250 needs a little bit more TLC during processing, but the end results for anything up to a 12x18 inch print are comparable.
Personally, since you've already got the 28-75 I'd look at something like the DA15 instead of another zoom. Smaller, lighter, great IQ, no duplication. YMMV.