hps is much better for flower but you must use a conversion bulb, like this one if you have a mh ballast. it is much better just to have an hps ballast for hps bulb though
http://www.plantlightinghydroponics....products_id=90

unless you have a switchable ballast of course

edit:
hps and mh won't make much of a difference on the power consumption. they are both hid lighting and use comparable power. cfl and flouros cost less to run.