Like any old oldschool proper serial MIDI interface setup (you Do need to buy an external adapter, it doesn't come built in), the Amiga has not much in the way of problems with jitter and timing. It's solid as fuck.
There's many programs supporting it, from the tracker interface of OctaMED to Music-X to the super complex and still unrivaled in options Bars & Pipes Professional.
OctaMED will not need an advanced Amiga for MIDI. You only need a faster Amiga if you want to use more of the internal sample channels. For MIDI you should be pretty safe. I would only recommend Bars & Pipes if you want to go absolutely batshit crazy with MIDI. Lerning curve is steep. Program is a bitch to get running. I tried once and was traumatized, never again tried to work with it.
Depending on your location, MIDI interfaces could go from "easy to find and affordable" to "oh my fuck why do I want to buy this?"
Whether you choose an Amiga or Atari ST or even a PC with a serial MIDI interface, you should have no problems. I would also really consider using an old DOS PC running Fast Tracker or something like that as it owuld probably be easier to source and work out than an Amiga or Atari ST.
