I have had, and have both the OD and A833 in my 1970 Cuda, 440 with 4:10 gears.

I put the OD in while waiting for the A833 (yes I was on the list for four years, not a big deal to me). While the gear spread on the OD, from 1st to 2nd, is not an issue for my setup it could be for lower horse power or torque cars. I liked being able to cruise around town in 3rd gear, and then have 4th when I hit the highways. The OD was in the car for over three years and I never had an issue with it.

The A833, gives me the best of both worlds, stock gear ratios from 1st thru 4th and 5th overdrive. In 5th I am at 2200 RPM at 65 MPH. It was an easy install with no cutting. Sense mine was a later A833 there where no break in noises, like some have said. Shifting is more firm than a stock trans, in my words, a more positive feel when going into each gear. I have not raced mine, I have gotten on it hard in street driving and shifting between gears is fine.

I have never owned or driven a car with the other transmission, so I cannot speak on how the A833 differs from them.

The 18 spline is noisier because of the pitch of the gear teeth is different than the 23 spline.

Seems some think these metal gear boxes should be as quiet as say some nylon gear gismo.

Actually it's the angle of the teeth not so much the pitch that makes the noise. Straight teeth are the strongest and that is what you will find in a dedicated race transmission like a Liberty or Jericho. It's stronger but sounds like an old dump truck. Oem style gears are angled to reduce noise. Since the 18 spline Hemi gear sets are expected to handle more torque than a 23 spline they were designed with less angle than the 23s which makes them slightly stronger but somewhat noisier.