Should I use the quote form to express my interest in a job, or should I write to the client directly?

You should follow the client's direction. When none is given, use the quoting form. 

You should never contact a client directly, or call, if the client asks that you do not. Doing so reflects poorly on you, on the ProZ.com community, and results in an inconvenience that almost certainly ensures you will not get the job.

If ProZ.com staff receives complaints of a user contacting an outsourcer directly, against that clients' instructions, the user's profile may be limited or removed in line with job rule #2.2 that states that job poster contact instructions must be respected.