The EP35 Gigabyte boards have the Realtek 8111B LAN chip and the P45 boards have the 8111C chip. I am not sure yet (can't confirm) whether the LAN driver is built into the more recent kernel or whether you have to tweak anything to get wired ethernet working. The people here who reviewed the Gigabyte P45 boards should be able to say?
Edit: I will make a wild guess but the Realtek LAN drivers appear to show more promise than the Marvel LAN drivers. Since, the 8111B/8111C chipset seem to be mentioned together and the P35 Gigabyte boards (with the Realtek 8111B LAN chipset) don't seem to have very many issues with the ethernet LAN. I have a Gigabyte EP35-DS3R board and have yet to have any major ethernet issues and I even have a Windows XP partition and have dual booted with no problems...knock on wood.
As for the Asus P45 boards, the P5Q has the following LAN info:
Marvell 88E8056/88E8001® Gigabit LAN controller
I am not sure if it's working 'out of the box' but from what I've read, there are issues but it's hard to confirm or know what the current situation is because of the different dates the posts/claims show.
This gentoo page has some info on it:
I'm assuming/guessing that what applies to that will also apply to Ubuntu and probably other distros too. At least, in time, it will.
I'm thinking of getting a new board but it needs 6 SATA ports. I was looking at P35 ICH9R boards but they're the same price or even more than entry level P45 ICH10/R boards so I thought I might as well go for the P45 boards. I am not sure about P43 boards. Are they any good? I was looking at Linux compatibility with P45 boards and I'm still unsure about LAN and sound issues. I hope that someone from Phoronix can confirm which board (Asus or Gigabyte) is probably easiest to use based on the different chips. Eventually, the drivers may be built into the kernel but if one is with less issues or has less problems arise, I'd probably go with that. I am not one who likes to build modules or tweak stuff like that. ;-)