DTA is less expensive, certainly, although you pay the same premium every month as the cost is rated across the term of the loan - they don't get cheaper as the sum assured reduces, sorry!
I also took out permanent health insurance (which does cost) as if I was out of work due to illness for more than six months, I couldn't afford to pay the mortgage otherwise.
I can't see the point of taking out more cover than you need - if the value of the mortgage should increase, you can always top up the policy, I did, it only cost an extra £5 a month for me.
By the way also, you don't have to take out the life policy with your mortgage vendor, you are free to shop around to get the best deal. You may have to note the name of the mortgagors on the policy but you can get the cover where you like.