I feel that prog metal shouldn't really be in a box. Isn't the whole reason behind it to leave space for musical experimentation? Very few get it right. I think symphony x might be one of the best at putting metal and progressive together. In such a non strained way. Some bands try way to hard, the tension and stress they went through to make it sound complex makes elastic bands everywhere snap under the pressure of listening to said albums!
EDIT: I don't really categorize prog music anymore. I just refer to it as all Prog Music. Prog rock, prog metal, its really all the same thing. Saying you are prog rock, or prog metal is really just going against everything prog is supposed to be!
Opeth for example is as much prog death, as they are prog soft rock depending on the album you listen to. Just saying! See Opeth-Damnation for reference!
Even symphony x for example, you can't put Walls of Babylon into the same category as Candlelight Fantasia. I mean what makes metal anyway? distorted guitars? I think not!