I have several varieties, and think the MRC is a good option to add sound for a modest price.
My experience with the 3 MRCs I've got (all steam) is definitely not that of Capt. Turk. I have them in the tender, with no "sound holes" or anything, and the sound is clear and certainly loud enough. Mine have 3 whistle options, not two. Option 1 is a heavy steam whistle, option 2 is a light steam whistle and option 3 is a european-sounding whistle. All sound quite good. Chuffing, steam, etc. all sound quite good.
Yes, the soundtraxx ones I've got are "better" -- but not dramatically so. The sound is perhaps a notch clearer or more precise, but certainly not enough to make one go "wow! what a difference!". Fact of the matter is I won't be too quick to spend the extra $$$ for soundtraxx in the future... Though I've ordered the shay version as there's simply no other option I know of.
The best one I've got is actually the "Quantum" that came in my BLI K4 - sound is just stunning, and it's the only one synch-ed to actual loco speed rather than throttle setting so it does slow down and speed up as the loco goes up and down grade, for instance. With the others (incl. Soundtraxx) if the throttle stays the same but the loco speeds up/slows down for grades, the chuffs stay the same... It can get particularly disconcerting if you have a heavy loco that really gets some speed going downhill.
To make a rambling post shorter, I'll summarize. Absolutely in favor of MRC decoders, especially if it's a situation of MRC-or-nothing. I would not MU two different sound decoders together... ANY two different sound decoders. They are always going to sound different and therefore funny.