At level 15 prayer you get 10% str and 5% attack, so in theory for the extra two-3 CB lvl you get from the prayer, you have to deduct 3-4 str levels and 1-2 hp levels, for an additional 5% attack and str. 70 str is 3.5 str lvl extra which you can get yourself
However with the base str, if you are say 4 str levels higher anyway with the 10% prayer, you are in theory 7.4 str lvls higher at 74 str = 81.4 str, as opposed to 70+15% = 80.5str.
So in theory, you are better with lower str, only thing you lose out on is the attack bonus which is only chance of hitting, with g maul it's inaccurate anyway and luck based so str is better