There is nothing technologically outstanding in a midi port, after all it is just modified serial port targeted at a specific standard. It is, and was, dead easy and inexpensive to either buy an Amiga midi adapter or just build one yourself.
And about the cpu (I guess we are talking of non accelerated computers), the Amiga had a slower cpu but it performed much better, because it could offload many of its tasks to its custom chips and dma engine which the ST couldnt.
The Atari had the midi port built-in from factory, so at first sight "it seems" more usable, but basicly an Amiga is better than an ST.
The Atari Falcon is of course, another story that needs to be properly disected