I run both and they both have their own pros and cons. I will say the 4R is far more forgiving after the fact as tuning it is all electronic outside of a decent rebuild and internal upgrades that should be completed etc. The AOD is going to be more affordable as you do not have to buy a controller for it, less the controller, both transmissions to be rebuilt and adapted to the D20 are relatively about the same in cost. The AOD will require you to do all of your homework first so you pick the correct hard components to change so it is strong enough for your use. I know some claim the gearing is better in the 4R however unless you are crawling it on the rocks, I seriously doubt most would actually be aware of the difference, not meaning it is noticeable by sound but by feel if both are built right its not as hugely significant in a daily driver or mild off road vehicle that I would warrant my sole decision based on that alone. The old school of thought was to upgrade the OD drum to the 4R to gain more surface area but with the advent of affordable kevlar bands and using the proper application servo along with boosted line pressure you can achieve sufficient clamping force without having to create a Frankenstein transmission, the aftermarket has addressed many of the old issues to date. That being said, if I was building a vehicle that would see deep water, harsh environment use I would opt for the AOD, mechanical is always better for harsh environment and reliability whereas electrical systems if not sealed perfectly can become problematic around those conditions. If you do not want to take the time, and it will be a lot of pre planning, it would be better to go for the 4R since after the fact, any adjustments to its driving characteristics can be resolved with a few clicks of a mouse whereas the AOD would require internal changes in the valve body or tail shaft to change spring pressures etc.
