I do both. Sometimes I do not want to make the thinking effort of designing and just want to get to the actual production part.
Sometimes I will confess I buy a pattern just becasue I am too lazy to figure out the fabric requirements. There is always that lack of confidence in my own math ... and I hate getting almost to the end .... and runout of a particular fabric in the quilt.
Some of the complex paper pieced patterns ... would take me FOREVER to draft the pattern.... and then there is that confidence issue ... will the components really all fit together??!!! and then that fabric requirement.... what if...
You may want to consider using non - copy or even copy blocks as just a starting point, you can modify to create a new block. Sometimes its easier to have a starting point.