There are so many different variables to play with, the real trick is not to let them confuse you if you haven't even built the most rudimentary of bases yet. I know lots of more 'advanced' trainers who will alternate rep ranges each session, or try to incorporate some high rep and some low rep work (with different exercises) in each training session, but they've all gone through the "stay with something long enough to see progress" phase by now.
Any change you make in your training has the potential to have a training effect. If you're lucky, it will be a positive one (if you're smart enough to give it a chance to work that is). I think you could adjust your exercise selection, your rep ranges, your rest periods,.. hell, you can get as crazy as you want, but pick ONE variable at a time, and see what happens.
Personally, I find that some exercises just lend themselves better to lower rep work, while others, usually because of the injury potential involved, or just because I like to build up a nice burn in the muscle, lend themselves to a higher range. No one can tell you what will produce the best results for YOU. Just be smart and analytical about what you do, and eventually, you should figure out your answer.