Many times I am asked which media is best for plein air (on location) painters: oil, watercolor, acrylic or pastel? I think there are advantages and disadvantages of each. I love oil for the creaminess and the smoothness of blending. Of course, oil is messy, dries slowly and takes more equipment. Watercolor on the other hand, uses the white of the paper, dries quickly, less weight to carry around and is the least messy. Then there is pastel, which requires paper and pigments, usually an easel and in my case, can be messy. Acrylic would seem to be the most ideal medium to take outdoors since it mixes with water and dries quickly. It can be painted on paper or on canvas, most likely you would carry an easel.
I have to confess, I am bias towards watercolor and oils. I don't mind having green fingers and smudges on my clothes from the oils. There is something I like about blending the edges of distant treelines and smoothing the clouds in the sky. I like the spontaneity of painting with watercolor on paper because you can be so portable with it. I tend to draw more for the watercolors and sometimes that can be a problem if you are pressed for time.
I have tried acrylics outdoors and just can't seem to adjust to their quick drying time. I also find that the colors dry darker, which is a problem painting outside with any media, except watercolors, that tend to dry lighter, I think. Pastels are the purest pigments so their color is the truest. It just seems that you need a good many colors with you. If you don't know what you are doing, you can create a lot of "muddy" colors by overpainting too much. They are dusty and you have to be careful how you transport them.
So I think the best solution is to match your personality to the media. If you are used to sitting down in your studio, then plan to take a chair and an adjustable easel. Otherwise, invest in a sturdy easel that will hold the size canvas or board you prefer. Try working outside in your own backyard first or a nearby park where there is shade and flat terrain. Once you get accustomed to packing your supplies, the rest is just painting!