There is a place in our spiritual life for both.
With routine prayers, the words have already been written or documented by someone in the past and have stood the test of time. They are good, honest, from-the-heart prayers. These include standards such as the Lord's Prayer, given to us by God's son himself. It includes simple childhood prayers like Now I Lay Me Down to Sleep, and many more. Each religion or faith has its own.
Sometimes, after we have said these words over and over, through the weeks and months and years of our lives, they become so routine we really don't even hear the words we are saying. At those times, if I realize I have been praying without attention, I try to vary them.
One way is to change the emphasis on one word in each sentence or phrase. So, instead of saying Now I lay me down to sleep, for example, someone could say, one night, NOW I lay me down to sleep, and another night, Now I lay ME down to sleep. Or, one night, we could say God BLESS everyone, and another night, God bless EVERYone.
This small change sort of wakes up the brain and the soul. Suddenly, we're thinking once more of the words we are saying.
Father, the mere fact that I take the time to pray is, I hope, pleasing to you. But please accept these small changes in the way I pray as evidence that I truly care about what I am saying at these times.