When you say that MMO is working "better" for you, what is your definition of "better"? In your particular case, it would seem to be hard to draw any conclusions regarding MMO's efficacy compared to ARX, since you're using MMO in engines that have already completed an ARX clean/rinse cycle.
From reading people's accounts on here that MMO keeps their engine clean, it would seem that MMO is a very good product, particularly on a cost/benefit analysis basis.
I will say that your experience with ARX is contrary to my own. I don't think that either of the cars that I've used it in were sludged or extremely dirty and the in both cases the filters came out looking not much different than on previous non-ARX changes. However, in both cars I observed increased and/or normalized compression across cylinders, so I'm fairly certain that some cleaning was done. Perhaps an MMO treatment would have yielded the same results (and I regret not having tried it first, but it is not available in Canada) for much less cost.
When I added 16 ounces of MMO to my oil 500 miles before my OCI the car just felt like it had more power and it was more responsive. I remember when I changed the oil that had MMO in there and it came out darker and the oil poured out faster. I have a drain plug with a magnet on there, with arx there was always a little bit of black goo on it, when I drained the oil that had MMO in there it was spotless. When I did the oil change I did not add MMO back into the engine as Demarpaint had told me too add MMO again 500 miles before my oil change, I took the car for a ride and it did not feel as responsive, so I put the car back on the lift and drained about half a quart of oil and added 16 ounces of MMO and the engine seemed very happy.
The only times I noticed a difference with arx was when I did an oil filter change and about 800 miles into the treatment.
I did a Clean and Rinse with arx on my Marauder and I noticed a difference in this car when I added 12 ounces of MMO 500 miles before my oil change during the Rinse Phase. Again the oil that was drained out did come out darker than any previous oil change on this car.
On the El Camino arx did not clean up the small amount of crud in my cylinder heads after using arx for 24,000 miles, but MMO cleaned everything up in about 7000 miles.
If my El Camino with 367,000 miles at the time and my Marauder with 33,000 miles at the time were clean then why did my oil come out darker after using MMO both times, my observations tell me that since MMO is thinner than arx it got into places and cleaned things up that arx missed or just could not get too.
Everyone of us are going to have different experiences with whatever oil additive we use since our engines and our drivng styles are not the same.
I believe products like Neutra, Rislone, and MMO are going to clean the ring packs just as well as arx. The disadvantage to arx versus these other oil additives just comes down to price, if a cheaper oil additive does the same thing as a more expensive oil additive then it makes sense to try a cheaper oil additive.