The S590 has more Horse power and higher Rated Operating Capacity. It is 66 HP & 2100 lb. ROC, where the S185 is 61 HP & 1850 lb. ROC. S590 replaces the S205. S570 is replacement for the S185. Besides the price difference, how much power and lift capacity due you need?
The M series (S5XX models) have more room in the cap and better visibility, especially to the front seeing bucket cutting edge.