How good is the AMD Athlon II X4 631 CPU?

Both are fine and both are for Socket FM1 motherboards. Is that what you are using or plan to use?

What make/model of motherboard do you have (want to use)?

Huh?? I didn't know there was a socket FM1 Athlon II.

I would get the A6-3650 since it is based on a modified Athlon II core (a little faster) and it has an integrated graphic core which can be hybrid crossfired with up to a Radeon HD 6670 to provide a little better graphics performance. Hybrid Crossfire only works with Radeon HD 6xxx series cards; anything faster than a Radeon HD 6670 and the integrated graphics core is automatically disabled.

The actual price difference between the Athlon II CPU and the A6 APU is pretty small since you need to pay shipping for the Athlon II while the A6 includes free shipping.

The 631 athlon is based off bulldozer and has lots of overclocking room. If ur not going to use the A6 igp with a gpu for hybrid crossfire prob the athlon wins
Forgot that the 631 is the low end athlon fm1 cpu, maybe try for a 641 or 651 difference is 2.6 in 631 2.8 in 641 and 3 in 651