One of the problems students have in a plein air workshop is trying to make their paintings look like a studio painting. For me painting outside and painting inside are 2 totally different approaches.
When I'm painting outside I'm in practice mode, more so then in the studio. I'm gathering material for larger studio pieces, trying different approaches to color and brushwork, and responding quicker to what the light is doing to my subject. Detail is at a minimum. I do better if I set a time limit to keep things simple and to the point.
Studio painting is more methodical. I spend a lot more time composing and recomposing my subject, more so than outside. Design becomes really important, it's what carries the painting. I will also give more thought to a color scheme or mood that I want to create. Outside is more about responding to what's there. Work in the studio generally has more broken color and more attention to edges.
The big difference is the atmosphere, being outside and seeing real color as opposed to being inside with photo references and color sketches. It's a different thought process. I like both equally, they just have different purposes.