in the marklin catalog you mean? I think fx is the regular digital system, and mfx is the new systems stuff.

Hello,
in order to explain things, I think we must be more precise. When you see "mfx" in the catalog, it probably is about a loco. This means that the loco can speak both the old language (protocol) Motorola, with old 6021 and the like, and the new language, which I prefer to call mfx-protocol. The Moible Station and the Central station are both able to speak both languages, but if for example an old booster is put between central and track, the mfx might no longer be possible.
And the language is the clue to understanding.
- no sign means that the loco can be run by Motorola, but not much extra functions. Possibly light can be controlled, 15 or 80 adresses, 14 speed steps. Most is enough IMHO, except there are no extra functions. The decoder might be a cheaper one, with bad slow speed behaviour, but you can't tell.
- FX means that there are up to 4 functions f1-f4 beside directional light. Still Motorola only, which is no limit. There is reason to believe that run behaviour is good.
- MFX means that there are up to 16 functions in total, 9 if run by Mobile Station or 6021. Most of those we've seen are just different sound effects, and are heard spontaneously also if no button is pressed. Mfx locos are easier to start play with; as soon they are placed on a track controlled by MobStat or CS, thet make themselves known to the central. But thet also tend to be more sensitive to dirt on track (which they share with some FX locos).

I hope you can sort those words out that you find too technical, and keep some. On problem is when you do this, that different people would summarize in different ways, possibly different from your needs. If you ask Märklin for example, they would say that mfx is the real new thing, new technology etc, a must for the future, just as marketing people tend to do. But there is very little new, very little needed in mfx IMHO. If the market will adopt this mfx protocol however, you might in 10 years or so, get a situation where Motorola is no longer supported, not because it's bad but because it's not new. But we haven't seen the success of mfx yet.

Regards,
Lars