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If I was getting paid to do a wedding and was limited to ONE lens & ONE body, I'd borrow my friend's 5D + 24-105L.
In Pentax land I'd choose the Tamron 28-75 2.8. But honestly, I would insist on the following setup:
Body 1 - DA*16-50 2.8
Body 2 - Tamron 28-75 or DA*50-135.
At least one flash, ideally an assistant for off-camera flash. Laptop. Extra batteries and cards.
After all, you're getting paid - and you really shouldn't screw this up.