I blew up the photo a bit and it's pretty grainy. I was expecting to see 1851's in both hands, but the one is his left hand has not only a round barrel but that sweepingly graceful ramrod link to the rest of the frame. The one in his right hand deals in 90° angles like the 1851. The one in his left hand doesn't. I have always like the looks of the 1860 Army better than any of the other colts of the era, and that looks like an 1860 to me.
I don't actually own one of those but I was given a non-firing mounted replica of both of those for Christmas about 10 years ago or so. One of these days I'll get a "real" working version of the 1860 Army of my own...
Twisted_1in66
Dan