The 6. You only need the 8 if you have more then 6 cylinders in the car.

I've been in contact with Keith at Flyin Miata and they seemed to have a negative experience with the FIC saying that the O2 modification wouldn't work properly.

Any reason why you think they didn't have enough success with it?

It could be they forgot to put the resistor in it. For most cars you don't need it, but apparently the Mazda o2 needs to have an inline resistor. It could be they were using an older version where the install manual didn't cover that.

Mine's running now on my 97 M3.
AFM datalogging works fine.

For posterity: pin part number is 43030-0003

If it was sold before some recent firmware update, 5 wires will be missing, which are for 3 new analog channels (Aux in, and analog A/B/ in/out).
They will send you the pins and wires if you send a prepaid envelope to them addressed to:

RMA FIC
Advanced Engine Management, Inc.
2205 126th Street, Unit A
Hawthorne, CA
90250