Nasi Lemak or Midin Belacan – which one belongs on your board?

What does each pedal actually do to my sound, and what is the biggest audible difference between Nasi Lemak and Midin Belacan?

Although both pedals provides a certain amount of boost in the mids region, their characteristics are totally different. Nasi Lemak adds in body, which then make your tone sounds thicker and fatter (hence the name), where Midin Belacan pushes your amp so your instrument can be heard better in the mix by emphasizing the mids and tightening the bass.

What kind of guitarist, amp setup, or playing style is each pedal best suited for?

Neither pedal requires a particular guitar or amplifier, but they solve different problems. Does your guitar tone sound like a empty tin can without much body? Nasi Lemak is your choice. Want to "push" your tone so people can hear you? Can't go wrong with Midin Belacan.

If I already own an overdrive, boost, or similar pedal, what would either Nasi Lemak or Midin Belacan add that I probably do not already have?

If the boost or overdrive you already have works for you, then there really isn't much reason to add another one. What matters is whether it actually solves the problem you're having with your tone. If it only gives you more volume or gain, but your tone still feels thin or gets lost in the mix, then that's where Nasi Lemak or Midin Belacan comes in.

How would I actually use each one on a pedalboard — always on, for pushing another drive, for solos, for shaping an amp, or as the main source of gain?

Personally I'd recommend using the Nasi Lemak as a always-on pedal and in the beginning of the chain – where it really shines. Some people use it as a solo boost too. As for Midin Belacan, I recommend using it as a switchable boost and a drive/amp pusher. Not that it doesn't excel being the main source of gain, but most people find it better using it as a secondary boost for the extra push.

If I can only buy one of them, what problem or limitation in my current rig should make me choose Nasi Lemak, and what should make me choose Midin Belacan instead?

It all comes down to what do you need, hence knowing where your tone needs fix is crucial to decide which pedal to go with. A general guide is if your tone sounds thin, go with Nasi Lemak. If your instrument can't be heard in a band mix, go with Midin Belacan.