I'd suggest spending the extra money on the Army. I personally have the brass navy (in .44, which actually an Army, so I guess it's technically going to be called a Brass Framed 1851 Army... maybe?). It was actually a hand me down from my grandfather, so I didn't have much choice, but my next BP is either going to be the 1860 or an 1858.
The steel frame, especially in the open top design, is going to be a more durable gun and hold up to heavier loads longer than the brass frame will. It will also be more historically accurate, as the 1860 was a .44 Army revolver.
I used to prefer to looks of the 1851, but the looks of the 1860 are growing on me. I used to hate the weird "rounded, amorphous look," but now I've grown to appreciate the curves and lines of the 1860 Army.
The grip on the 1860 is slightly different from that of the 1851. I prefer the 1851 grip, myself, and it is actually more similar to what you'll find on an 1873 than the grips on the 1860. I've heard that the Pietta (the Brass Frame 1851 .44) grip isn't accurate, either. So, you might want to take that into account, too.
There's a lot to weigh out, but it really comes down to: what do you want?