Which of these brings more traffic to your blog?

[Justin]

I would stop right there and get rid of the idea of targeting "weight loss" way to competitive and could take years to rank for.

Find a sub niche of weight loss like, weight loss for women, weight loss for kids, how to lose weight fast, weight loss for seniors.

I would imagine that these are still really competitive, find a sub niche inside these or something similar.

I would focus more on a general domain name for weight loss, exact match domains are useless nowadays anyway, do focus on something catchy people will remember.

I would start with long tailed easy to rank for keywords and progress up to the tougher keywords once you gain authority and traffic.

You could always use these long tail keywords and the traffic from them to funnel them over to the much more broad competitve keywords.

I use Long Tail Pro for finding perfect long tail keywords and determining how hard they would be to rank for, much easier than math recommended above.

Good Luck