Here's a thought about different brands of similarly-sized mpcs, in this case the Bach 1 1/2G: perhaps they are analogous to different recordings of the same piece.
If you have several recordings of one piece, they all differ from each other. Yes, they're the same piece of music, whether it's Tchaik 4, Sibelius 2, Debussy's La Mer, Bruckner 4, Pictures at an Exhibition, etc. . . . However, each recording is by a different group of people. Sometimes it's the same orchestra but 20 years later, or with a different conductor, and sometimes it's the same conductor but with a different orchestra. All the recordings play the same music, but the end result--the product for the listener, that magical moment captured in time--is different in each case. Not better, just different (excluding clear performance errors like chips, clams, etc.). Some prefer Maestro Smith's version of Davis's Symphony in QB with the PDQ Orchestra, while other prefer Maestro Jones with the XYZ Orchestra, and still other prefer Maestro Jones' older recording with XYZ Orchestra from 20 years ago.
If you listen to Stravinsky conduct Stravinsky, you're in for a sonic and interpretive experience. You might not like it, but it might grow on you. If you play a Mt. Vernon 1 1/2 G, you might not like it at first, but it might grow on you. You might find some qualities in the 1 1/2 G mpc that you like, but decide that you want a slightly different rim shape, so you try a Warburton, a Greg Black, a Rath . . . choose your maker. Just so, you might find that you prefer a different interpretation of that Stravinsky work, whether it's conducted by Solti, Karajan, Reiner, Chailly, Boulez . . . choose your conductor . . . or Cleveland, London, Chicago, Vienna, Berlin . . . choose your orchestra.
Not to stretch the analogy too far, but listening to different recordings is in some ways similar to the way that different shapes and characteristics affect the tendency of any particular mpc to tend to sound a certain way, whether it's the rim shape, cup depth or shape or volume, the shape of the backbore and throat, or anything else. What you play not only has to be comfortable (it has to feel good), but it should allow you to re-create the "sound in your head" (thanks to Charlie Vernon for the phrase) more easily and allow you to play the music in front of you. It's like Chris said a page or two ago: "A mouthpiece doesn't do anything. It allows you to do things."
So, pick your piece. None are right or wrong or good or bad (unless it's clearly a manufacturing error in the mpc), just different. Some work better for certain individuals or different musical applications.
I hope this analogy works for somebody . . . besides me . . .