For myself I pick a profession/class I like based on skills and playstyle and try to master it. Picking one based on difficulty (whether easy or hard) just doesn't appeal to me.
In addition, with modern games when balancing can happen often, I think it is a bit silly to pick something based on the current balance. Though I'm not sure what arena.net is planning to do with balancing professions.
edit: my main will be engineer, first alt will be mesmer.