^^ GA-880G USB 3 is not 4K in SMC, it is 4.8K. I think you are talking about the 880G-DH motherboard which is stripped down version as it has only 2 ram slots and lacks the USB 3.o support.
Now between GA-880G-USB 3 @ 4.8K and M5A88M @ 5.3K, I will go with the Asus M5A one as
i. M5A motherboards have full support for AM3+ processors from ground level, not by BIOS update as in the case of Gigabyte mobo.
ii. It has SATA III 6 Gbps ports, missing in 880G-USB 3
iii. M5A88M has slightly better build quality over and better VRM over 880G-USB3 and it supports CPU TDP up to 140W.
iv. SB850 Southbridge compared to SB710 in Gigabyte
The only point in favor for the Gigabyte motherboard is cheaper price and two PC-E X16 slots @ (X16-X4) mode, allowing the provision for Crossfire or dedicated PhysX setup.