The two practice sketches above are obviously very different. Each was done in the name of practice, but different skills are being practiced in each, and that is not simply because of the different media used to create them.
It’s important when practicing to know what skills specifically you are trying to develop. Are you working on color? the ability to see values? the ability to block things in quickly? or the ability to show detail? With a practice session, particularly if you have a limited amount of time to practice, it is always best to have a goal in mind for that session.
For example, the digital sketch at the top was done from a photograph I took of a toad that was on my doorstep. My intent with this sketch was: a) to better learn the ArtRage app on the iPad, and b) to focus on capturing some of the details of the surface texture on the toad’s skin.
The second sketch, done in acrylic on watercolor paper, was done from life. My goal there was to see how quickly and with as few strokes as possible I could capture the gesture of the toad, along with a sense of its form based on the natural lighting of an overcast day.
As you can see, I have 2 very different outcomes and worked on developing 2 very different skill sets in these practice sessions. I think the more diverse your practice sessions are, but the more focused (try to have only 1 or 2 goals with each session), the more well rounded your skills will become in general. This will ensure that YOU remain in control of how your drawings and paintings look. The more well versed you are at a variety of techniques, media and approaches to capturing what you see on paper or canvas, the more options you have for being expressive and creative when you are working on your finished paintings.