3yr ARM is better. assume that there will be QEn (n>=2)coming, fixed rate is not necessary. no mattter there is QEn or no QEn, short end will be low for a long time. look at Japan...got it?
30-year conformed 4.5%
15-yr: 3.75%
If you are young, 30-yr fixed will be perfect. In 10 years, inflation will literally clear or reduce your mortgage debt significantly.