Just my opinon, I would choose the FAL for 2 reasons.
1. From what I've read here, the AR-10 seems to be having some problems (though they seem mainly related to many bad mag conversions, not the gun itself), and mags are expensive M1-A mags to start with. You can get 4-5 new, perfectly functioning FAL mags for the price of one converted AR-10 mag. I'd rather get a new mag meant for a gun then a cobbled up, made to fit mag, that raisies failure concerns. If new AR-10 20 rounders were widely available, I would feel different on this.
2. Especially if I already owned an AR in .223, I would find it far more enjoyable to own a gun with a vastly different construction and operating system, rather than a slightly larger clone with pretty similiar parts and function.
Much more enjoyable to shoot, take apart and fiddle with, re-assemble, etc...