I do both, but for different reasons and at different times. I greatly enjoy both methods and the variety they offer. I seldom get bored as a result. I don’t use other people’s patterns at all so everything is from scratch either way.
I design quilts down to the finest detail when I want something in particular...a show quilt, a challenge quilt, a gift quilt, quilts from a long held idea or a dream I had...something that has definition before I start designing. It can take a few days, a few weeks, or longer to get everything laid out exactly the way I want it including which way to press the seam allowances. For a very few, however, I may have no idea at all how to physically construct it. That doesn't enter the picture until I actually start piecing the quilt...and it has sometimes been the end of it. Yes, I have actually designed quilts that I still haven't figured out how to build (and I am 4+ decades into this sport).
I also make quilts improvisationally when I want to play, learn, dare, relax, experiment, risk. These are usually technique quilts, art quilts, intuitive quilts and generally start with a broad vision of shapes and colors. The goal from the outset is nothing more than exploring the moment, an absence of definition if you will, in contrast to the design goal described above. This process may be more nebulous and spontaneous, but the result is no less accurate, intricate, or challenging than the first.