Gigabit LAN means that, theoretically, you can transfer 10x faster than the standard 100/10 bit systems (i.e., 1000 bps). However, unless you have a home or office lan that is comprised of equipment supporting and running 1000/100/10 (gigabit) components, you will be using 100/10. In short, it is highly unlikely that you will see any benefit from the gigabit chip.
Where benefits will be seen is when using a better lan chip. Some use less CPU load than cheaper brands. I'm not enamoured with the lan chip on the P4PE/L, but it works fairly well for now.
I would not base you buying decision on the lan chip of the 2 boards. Look at the other features on the other board (SATA, RAID, etc.) and see if you want them. If not, get the cheaper version. That's what I did.