I got a free breeding to this guy 

https://www.allbreedpedigree.com/rc+king+gold

I have two mares to choose from. 
Mare #1 https://www.allbreedpedigree.com/bonanzas+countess

Mare #2 https://www.allbreedpedigree.com/solanos+kid+cody

Of the 2 mares, bloodline wise which would be the better cross? It will be used for a working/general riding horse. Mare #1 is 19 15.3hh and has 1 foal.  Mare #2 14hh and is 14 and maiden. Both are both pretty good confirmation wise. 2 is mutton withered and a little sway back but nothing terrible  

Mare 1

Mare 2 has a conformation defect (away back). Mare 2 is only 14 hands and first foals tend to be small. 

Most of the horses on all three sets of papers tend towards working cow/reining, right? 

I don't know much about the stallion other than King. I know the first mare has some halter breeding but other than that I don't know much about her. The 2nd mare is 100% cow horse top and bottom. 

I would go with the one that is not mutton withered.  That is a deal breaker around here.  Being sway back can be an indication of kissing spines.  AAEP has determined that kissing spines is hereditary.  

we're going with the first one. And good to know about kissing spine. I didn't know that.