I don't have all of the answers, but here's a few of the most important. They have many features in common, however...
(Note: Samplitude comes in three versions - Producer, "Standard", and Pro, which range widely in price.)
Samplitude is a full featured digital audio workstation (DAW) - MMM is not.
Samplitude can do (simultaneous) multi-track recording - MMM can record only one track at a time.
Samplitude has more tracks - MMM has enough for the average person.
Samplitude supports all MIDI functions - it's still not clear to me if MMM does.
MMM can only import/export at 16 bit / up to 48 kHz - Samplitude can import/export at 24/32 bit / at much higher sample rates and in more formats.
------------
Perhaps someone else can fill in any missing pieces. Unfortunately, they don't offer a direct comparison chart for all of their music products. However you can look at the "Product Info (PDF)" on these webpages for some info...
It's isn't a question of "better", it is a question of what you need out of a program. There are a lot of differences, and many overlapping features, but fundamentally they are different programs. I think of it this way, which might not fully explain it, but it helps draw the division between the titles.
I believe they both share the same MIDI editor, which MIDI is a whole other topic in terms of music creation, so we can just leave it that both have it.
Music Maker
Music Maker is a loop arrangement program (piece together samples based on BPM [beats per minute] to create a song). Music Maker also allows you to record a single track at a time, and allow you to do a playback while recording so you can create a full song using it. It does not offer you the robust editing as you will find as part of the Samplitude software, but I believe you get the Music Editor with the Music Maker program as a separate outside editor, this will give you some of that capability.
Samplitude Producer
Samplitude is DAW (Digital Audio Workstation) a multi-track recording/editing/mastering/authoring software that allows you to do multi-track (up to 8 track simultaneous) recording (provided you have the gear and computer that can handle that task) with a full, high end fully functional editing, effect suite. You cannot easily do loop arrangement with Samplitude, but it is possible, but it wasn't designed for that.
Samplitude requires more experience with software, recording, editing, and mastering to fully appreciate the program so in short, it has a steep learning curve, not because the software is hard, but because this is a music production suite software from start to finish, it is not a sit down and make music by building it with loops and virtual instruments software.
Procyon below has given you some other information, but I recommend that you try the trial versions.
I now see that MMM17P perfectly fits my needs. I've no equipment for recording multi tracks - much too expensive – and I’m a “one-man-band/song-maker” – with no band to work with. I'm very fond of working with loop as well. As far as I can see the MMM-loop-station (or what you call them) is perfect to me. There are some software synths in common concerning Samplitudes and MMM's as far as I can see.
I've today got MMM16P for uploading softsynths from the net - free or not - or from DVD's attached to Music Magazines, as well as fx's - easily uploaded to MMM16 and for my sound experiments.
I've bought MMM17P which I intend to keep free from other synths, because I noticed some dysfuntion when it came to mixdown when MMM was loaded with other soft synths and fx’s then the ones built in.
Thus, I use 16 for playful sound-, music and loop-experiments and if the download-function doesn't work when I need it I can catch the sound with a streaming sound recorder on my computer or with my Korg SOS connected to the earphone plug. The sound isn't perfect always working that way to the sensitive ear – but good enough to express my thoughts, ideas and feelings through my music. There's always a solution when something happens to dysfunction.
And then I keep my 17 free from "alien" synths and fx's to let the mix down function work without problems - the sound is of course always better then - 'cause sound quality is not totally redundant to me.