Neither is better, just different.
A 15 year loan will be paid off faster, may be a slightly lower rate, and you will definately pay less interest. However, your monthly payments will be higher.
A 30 year usually will have a higher rate, although not much higher, you will pay it off in 30 years, and you will pay more interest over the life of the loan. The upside is that your monthly payments will be lower.
Here is a free online tool you can play with to see the difference. You may want to zero out the PMI and Tax fields before you start so that they don't skew your results.
There are also many other options to consider. Contact me if you would like a referral for a great mortgage professional.