My mother bought me an oil painting kit while I was in my early teens and had yet reason to consider other possibilities. Until that time I was perfectly happy with my pencils and crayons.
In high school I was introduced to acrylics, pen & ink, scratchboard and other drawing and sculpting materials but continued to find the versatility of oil to be convenient and liked the tactile qualities that imparted quality and value to me. I loved the richness of color, the deep luminous effect of darks, the physical build up of opaque whites and the feeling of permanence.
In art school I struggled with watercolor and longed anxiously for my sophomore year to arrive when the painting requirements would switch to oil painting.
My first fulltime art job took me to American Greetings and had to learn/use the medium of choice for commercial studios, Designers Gouache. It was also my great fortune to be thrown together with a number of watercolorists on their way to becoming award winning members of AWS. Robert Laessig, Alan Chiara, and
Morten Solberg among them. It was an experience much like Michael's but multiplied.
I don't do many watercolor portrait commissions (there is one on my website) in part because they are not as predictable if you do them in my style (see
my post on edges ). As Tim suggested, the pricing expectations are lower as well.
I enjoy pastel a lot but find it difficult to price fairly. (I'm probably not telling you something that you have not already experienced). Dan Greene's pastel workshops are great. Take one if you can.
While oil is my first choice of media I have to say that I have gained a lot by working with others. I know that I am a better oil painter for having the experience. And I think it worth mentioning that the likes of Sargent, Wyeth and Kinstler display considerable skill in watercolor as well as their medium of choice.