Take the 30 years when the bank still allows you to get such long tenure.

No matter how you compare it, the 30 years option will be better than 20 years option. Even though on surface, looks like the final repayment differ a lot but after 10-15 years, that amount different after taking into account of the inflation will become less significant.

You won't see it now because you have yet to see how value of money depreciate over time.