I prefer 1/4" Baltic Birch Ply for portraits but i have used 1/4" and 1/2" Pine and had some good results, i've also tried 1/4" MDF but it's no good for fine detailed cuts as it's very weak, and it really needs to be coloured when finished, it really all depends on your project and what sort of end result you want, thin wood can look a bit flat, thicker wood adds depth and shadows, try experimenting with different woods and thickneses, you might be supprised with some of the results...
Ian