a500 or 200r4 ?

If your a mopar purist that's fine. But my ride is a " Frankenstien" . I would like to install an automatic Overdrive transmission. No matter what transmission I use, I will need to make an adapter plate. So using an a500 or a 200r4 does not change the adapter plate problem..
From a stand point of which trans would be better to use, which trans would have the least parasitic power loss ? And which trans would be better in a street driven car (with less than 250hp) ? What would be you choice ?
I've heard that the a500's OD section is working all the time, even it lower gears (but not sure about that). So would the a500 suck up more power "all the time" just asking.