Low voltage fixtures are more expensive initially but they have lower life cycle costs.  They also produce a nicer light, with warmer, brighter and more vibrant colors.  There are many different varieties of MR-16 bulbs to choose from. The bulbs are longer lasting, they typically have a 5000 hour life or better.  You will get better energy savings with this technology. 

To summarize, the lifetime operating costs for low voltage lighting is lower due to the energy savings and less frequent bulb replacements.  However, they do have a higher inital cost.