Between these two, I'd say go with the XTi because it has a larger image sensor. But the XTi is a really really old camera (2006 is old in DSLR terms). So unless you're getting it pre-owned for super cheap, I'd recommend something newer like the T3 or, if you ever plan to do video, the T3i.
Another reason to select the XTi over the DMC-GF3 is that the XTi has a flash hotshoe and the Lumix doesn't. Although they both have built-in flashes, if you want professional-looking photos at an event, you'll likely want an external flash. On the other hand, if you're planning on taking staged photos and will have access to lots of sunlight then the hotshoe is much less important.
One last thought, if you're in a rush to get a website up with professional looking photos consider bartering with a photographer at one of your catered events (I assume you cater some weddings). Someone developing their portfolio would probably be happy to shoot your food & decorations for free.
