Which would you choose and why?

Diet plan and exercise, without a doubt. Here's why:
Diet pills are unnatural, they are chemicals made in a lab. If there is a natural alturnative (eating right and exercising), that's always preferred.
Although you'll typically see results from diet pills faster, they can become addictive and have side effects. When was the last time you heard of someone having unhealthy side effects from exercising or eating salads (okay, save for the recent e.coli problems, but that's an issue of contamination, not a side effect from the actual vegitables)?
Plus, wouldn't you feel better knowing that you're losing the weight and getting in shape as a result of your own efforts and disipline rather than just popping a pill? People rely too much on instant gratification these days and don't think of the long term effects. Fix it pills are only temporary fixes and usually cost us in the long run.
Although it will take longer and require more effort on your part, I strongly suggest dieting (properly and in a health-conscious way, don't starve yourself) and exercise.
Pills are bad, 'em kay? =)

diet plan and exercise. this is the only healthy way TO BE HEALTHY and have the body we want. some of us are too lazy to get off of our asses and exercise. dont chose the weight loss pill...
=D