A metal D vs metal M I don't know.
A metal D vs a hard rubber M, I do know. The metal D is a little brighter and thinner top to bottom. The hard rubber M has a little more guts in the bottom and presence in the top. Granted, one is metal, one is rubber - two different animals.
On soprano, I play a metal D6.
I play a Yamaha Custom 875 (silver) alto, I have metal D9, D10, and hard rubber M8 mouthpieces. I love the hard rubber M8 better than the metal pieces. I bought the hard rubber while studying music in college and paid about $40 for it in 1991 (another story, anothe time). I played the Dukoffs for a few years before I got tired of cutting my lips when they would get pinched between the reed and mouthpiece where the sides of the mouthpiece rounded too much to the rails. I have used cane and plastic reeds -- same results. I have the metal ones put away.
The last metal Dukoff mouthpiece I got (in about 1999) actually had a pin-sized hole in it from a manufacturing defect. Living in Miami, FL, I took it to Bobby's factory in Kendall by the airport in a warehouse area, met Bobby Dukoff in person, and after he realized it did get past quality control, he brought out 5 other mouthpieces for me to try and I selected the best one. It was an honor to meet a legend. They had stopped making the hard rubber mouthpieces at that time due to costs - so they told me. Personally, the quality of the hard rubber FAR surpasses the pop-cast metal used in the metal mouthpieces. The inside of my metals ones looked like moon craters. I even asked Bobby about the rough finishing on the inside of the metal mouthpieces and it affecting the sound quality or intonation. He became defensive and said it had no bearing. He may be right, but I personally think the Dukoff quality has gone down. The metal tarnishes to a dull grey after it gets exposed to your first gig. The metal is so soft you can dent it with your fingernail almost, and none play alike.
The M in hard rubber gives a great sound with a little more guts in the low end and doesn't make the upper notes thin. The response on the M (HR) is great and altissimo is clear. The table is well finished, even rails and tip, chamber is well finished, and sound quality is a cross between a Beechler Diamond Inlay (HR) and a Vandoren Jumbo Java (HR). I love it and if I can find one in a M9 or M10, I'd buy it.