OK, this is a tough one.
One of my big breakthroughs, of late, has been singing more in my practicing. Thus I plunge into that old debate...
Sing in "fixed do" (C is always DO no matter the key)
Sing in "movable do" (The tonic note is always do no matter the key)
I don't really have perfect pitch
A lot of music I'm working on is chromatic with shifting tonal centers or ambiguous tonality
So this means fixed do. Right?
Here's the thing: the syllables only help if they help you hear the note in your mind.
Since I have less experience singing solfege, I still have many moments when I can hear the correct pitch but hesitate trying to think of the correct syllable to use.
Obviously, at this point solfege ain't helping.
For the time being, I'm trying singing in fixed do but using a slightly altered system:
I replace mi and ti with "may" and "tay" because, if I ingrain this system in my brain, I don't like instinctively thinking of the "eee" syllable when I'm hearing these notes in my head.
Or perhaps, I should just sing everything on "la" or "ba"
OK, enough rambling. Here are some other websites that provide intelligent discussion..
A blog entry by Scott Spiegelberg of DePauw University (some good comments below the blog entry, too)
An interesting article by Jody Nagel
I found a very long thread of comments on the topic at violinist.com