Using 28-135mm for outdoor shooting

Hi all! I'll be shooting a small outdoor wedding in September. I have the 28-135mm IS.. And was wondering if it is worth getting the sigma 17-50mm 2.8. I know that the sigma would work out in low light situations..but It being midday..it shouldn't be an issue. Im not sure if it's worth the 700 dollar price tag for a range I have covered. But at the same time, I'm wondering if that extra stop is worth it in terms of depth of field.

I'll be walking around with a 70-200L f4 and an 85mm1.8 as well.

Any suggestions? Thanks a bunch!

I think the most important thing for your consideration given the location and timing is FLASH. Even if you merely use on camera fill flash you will see much better results in harsh mid day sunlight.

Your setup will work fine. Flash fill is a plus though.