what i have heard from PD's: step 1 is used more universally, because many students have not taken step 2. that being said, if there is a step 2 score, it is used more, because it is much more pertinent and relevant to the material you need on the wards.
so, moral of the story: do as well as you possibly can on both. if you have a weak step 1, be sure to do better on step 2. if you have a great step 1, consider waiting on the step 2. but, no matter what you have, make sure that you are a hard worker, have great LOR's and can present yourself well on an interview. the programs are going to hire the person, not the test scores....you have to live and work with these folks for several years!