mars is less expensive for more watts and similar efficacy.. unles the smaller light is perfect-form for area and need, i'd go with the mars light in this case.
i did a huge writeup for someone else explaining in depth in last month.. search grow questions and you can find it... not re-writing that book.. but goes over coverage, efficacy, longevity etc.. all the aspects that you need to consider... while this choice is easy, sometimes you have to delve into all that stuff to know for sure.