I've been looking at the Gigabyte Z68 boards a lot lately, and this i pretty much what I've gathered:
UD3: Pretty decent, supports 2 video cards, plenty of SATA
UD4: Adds eSATA support, that's about it
UD5: More power phases, more USB3, can add a third video card
UD7: Even more power phases, even more USB3, supports 4 video cards. I believe it sports a NF200, so you can get 2 x16 PCI-e slots at once
So right now, there's not a whole heck of a lot of difference to justify the sometimes $100 difference from one board to the other unless you have something specific you're after.
The other distinction to make is that the first gen Gigabyte Z68 boards (Z68X in the model name) don't have Virtu and Quick Sync enabled, so they're functionally closer to a P67 than a Z68. However, the newer ones (Z68XP) are fully featured (w/ HDMI out). At this point only they've only come out with UD3 and UD4 versions, but I'm sure the 5 and 7 will soon follow.
For the record, I'm leaning toward a Z68XP-UD4 myself, unless they come out with the 5 within the next few weeks, and it looks like it's worth it.