I do both. Right now it seems that running help with my weight loss more than cycling. But I think that may be because it's newer for my body. I have been cycling since last Autumn, but only running since this past Spring. When I spent a Winter in the pool twice a week, THAT seemed to be really helping with my weight loss. It's cross training and whatever is newer to my body (hence developing under-developed muscle groups, etc) seems to be what helps most with the weight loss. That's why it's good (for me, at least) to mix it up. Plus, it keeps it interesting.