...and why?

Drag race stuff

Not even a contest, pick the 89981, it can do everything the 8998 can do plus coil on plug, crank trigger wheels and adjust the dwell on individual coils. The only disadvantage is it is not reparable.

FWIW...I had a misfire issue w/ a digital 6al-2 programmable box that tested fine w/ the 8998 tester.

Never used one of the MSD testers, looks like one might be convenient if you have the money. If you want to see if the box will fire the coil simply use a test light hooked to battery positive and toggle the connector to the pickup coil connector. I use an oscilloscope to check the other functions. As mentioned the tester will not check all the issues, and even a scope misses things sometimes when they happen under load in the car going down the track, but will check fine at idle in the shop.

That's what I was thinking, But the fact that the 8998 is almost double the price had me wondering what I was missing.

The ability to test and verify rev limiters, shifters and shift lights, nitrous solenoids, and anything else using RPM, along with the functions of the box, coil, wiring, etc., without having to run the engine in a race car, put laps on it, or throw parts at it, is where the value of these show up.

89981 for the reasons given as well as it works with the Grid which the older 8998 does not. It also allows you to simulate runs etc to ensure RPM functions work.

Yes Sir, I saw that. Just thought I was missing something. Better is seldom cheaper.