"Well it's not 1G!"
But it's not 10G. Anything on the local network that you want to read from or write to that is faster than 1G will almost certainly be faster than 2.5G. A single mechanical drive can outpace 2.5G now in sequential, and really you should be using an array which would go even faster.
If your motherboard comes with 2.5G, neat! If 1G or 2.5G is the only differentiating factor, just go for 1G. If you actually want faster networking and you're putting cards in, go 10G.
If you somehow have service from your ISP that is actually, for real, faster than 1G, then ignore this.