overpayment or shorter time; which is best?

Evening all,

I am about to remortgage and currently have 29 years left on my mortgage. I can definately reduce to 22 years, but if I stretched myself I could reduce this to 17 years.

Is it better to go for the 22 years (with the safety of lower payments should I come into payment difficulties i.e. loosing job) and overpay or just go for the 17 years?

Does it make a difference to the overall mortgage balance?

Thanks