For those of you trying to decide which one... there are A LOT of threads specifically on which ACT models suit which power levels and driving styles. There are only 2 pressure plates that would apply to our applications... the HD and XT for both the 1.6 and 1.8. And then there are 6 discs that you can mix in.
If you're making ridiculous power, like 350+, then you probably want to do some serious homework on your own and match a clutch to your exact driving habits and needs.
But for most of us, running between 150-250whp, the choice can get a little more complicated.
The general feeling is that the HD pressure plate feels just a little stiffer than stock. The stock Miata clutch is fairly light... and stepping up to the HD basically feels like what you'd expect to find on an average V6. The XT on the other hand is a big step-up in stiffness. I've heard it described as roughly twice stock pedal effort... essentially what you'd expect to find in an average aftermarket V8 clutch. Stop-and-go traffic with an XT might get a little old after awhile, but is still very doable... and after a week or so, most guys say they're totally used to it and going back to a stock clutch just doesn't feel "right" anymore.
If you're a small guy with puny bird-legs, maybe the XT isn't for you.
As for discs, here's where it gets tricky. The ACT website does a good job of describing what to expect between the different discs. The 4-pucks will be a true on/off switch... with the 6-pucks being a little more liveable, but definitely something you're willing to make a sacrifice in power handling vs. driveability for over the solid discs. Just be sure to do your homework and make a good choice.
Flywheels, you're on your own:
Either Miata engine size can use the clutch setup from the other size. 1.6 and 1.8's can use each others clutches as long as you use the flywheel for that clutch. The total weight of the rotating clutch/flywheel assembly is a little lighter for the 1.6 as 1.8. The numbers are out there somewhere... and it's only a small difference.
The advantage is that someone running a 240whp 1.8 can opt for 1.6XT setup... as the 1.8HD setup probably won't be enough... and the 1.6XT is a lot cheaper than the 1.8XT.
Similarly, somebody running a 200whp 1.6 might feel he needs more than the 1.6HD, but doesn't want to step up to the 1.6XT pressure plate. The easy solution is a 1.8 HD to keep the pedal effort and cost down.
The above examples would assume the solid discs, and not any of the puck models.
The decision on what model clutch is something you should put some thought into... but don't lose any sleep over it. As long as you're not severely underrating the clutch, you're gonna be happy with an ACT.
But no more discussion about this in this thread... start another thread if you're looking for help making a choice.