Low voltage lighting gives a much brighter output than mains voltage, when using the same wattage lamp.
MR16 and MR11 are two different lamp types. MR16 has a diameter of 51mm, and MR11 has a diameter of 35mm.
MR11 fixtures are generally smaller and more discrete than MR16, However you have a much larger choice of light bulbs when you choose MR16.