Help me settle this 20 yr fixed vs 30 yr fixed mortgage debate

[scotchnrocks]

Had a debate with a co-worker today on a 20 year fixed vs a 30 year fixed mortgage. For simplicity, assume a rate of 6.125% on both and a $150k loan balance. What rate of return on your investment would be needed to outperform the 20 yr fixed after taxes at the 10 yr and 20 yr marks by investing the difference in P+I of the 20 yr and 30 yr? Is there a calculator for this somewhere? Any comments on the risks and such of doing this ?