Thank you Jean for making me think about this. Good question!
I am blessed to be able to paint in both media. A lot of artists limit themselves to one or the other. I think it is a shame to do that. As a matter of fact I also love working with Pastels, Conte, Ink and any other medium I can get my hands on.
In his landmark book Understanding Media: The Extensions of Man, published in 1964, Marshall McLuhan proposes that a medium itself should be the focus. He said that a medium plays a role not only by the content delivered over the medium, but also by the characteristics of the medium itself. He wasn’t talking about paint media, but he might as well have been.
Each of these media has a personality and a spirit that is totally different from the other and must be approached in a radically different way. The obvious example is how you work oil paint dark to light and watercolors lights to dark of course, but that is only on the most obvious level. The way each medium acts and reacts to brush strokes, the way the colors are laid and how they function once they are on the surface is hugely different as well. Mostly though, each of them has a distinct character that must be respected if you hope for them to work with you in expressing something on a surface.
I have painted several scenes in both mediums at different times and the results are hugely different. You can have the same composition, the same color scheme, the same size painting, but the medium will impose its presence in a work and you have to respect that and work with it. The results are amazing. Like great poetry spoken in different languages, the music is individual and never repetitive.
Admittedly, some scenes lend themselves more to one medium or another. That is not to say you can’t paint a scene in both or either, it is just that your style and the scene beckon to one or the other medium.
Here is my latest Watercolor, and below it is an oil of a similar scene (not exactly but somehow similar). Most people use watercolors very faintly. I don’t like that. I charge my colors fully and let their personalities come out. I try very hard not to fuss with them once they are on the paper and that gives them vibrancy and life.
The oil is admittedly more moody in this scene, and the colors are meant to be more subdued than the watercolor above, but the high grass/ brush is in both. Anyway…
All to say, I am so lucky to be able to paint in more than one medium. In art schools you don’t specialize in one medium and there is a good reason for that. The more color languages you can speak, the better! I hope this answers the question in some way.