I have the latter and I am liking it, it overclocks nicely and is stable. But what the Z68 has above the P67 is the integrated graphics and the Hybrid SSD Smart response technology. Do you think you would use either? Probably not. So I would base my decision on other features like the peripheral connections and the price.
The VRM power deliver system is a bit different. The P67 has 16 CPU phases, the Z68 has 12 CPU + 4 iGPU phases. I doubt it would make any difference unless you did an *extreme* overclock.
You can get that front panel USB box for the Z68 for $30 from Asus if that matters (the Z68 has one for the back panel).
The P67 has been around longer (it has 10 BIOS revisions - yikes!), and it doesn't even appear on the Asus USA listing of P67 boards. That might only mean they are not organized. Their BIOS notes indicate that they will support Ivy bridge on it.
Frankly, the UEFI interface doesn't do much for me. Sure it is nice to be able to use a mouse, but it is also easy to get around with the keyboard and you won;t really spend that much time in the BIOS (after playing around for a while
BTW, the first Z68 I got had a motherboard issue with graphics interfering with sound. The second is great. I think, from any vendor and any model there can be problems - perhaps due to shipping and handling - I don't know (but just look at the newegg reviews). I got mine from Amazon, and if you get a faulty one, they will ship you a replacement and you don't have to return the item until sometime after you get the replacement. Since I had Amazon prime, I got the replacement in two days. So I would recommend this route with as Motherboard. The z68 pro is $199 there but the P67 deluxe is more than newegg.