A9xx evolved in to 42/44RH then RE.
A727trans evolved in to 46/47/48 RH/RE.

The numeric coding was adapted by most of the OE's to show number of speeds and relative torque capacity.

A500/518/618 are useless designations when you compare to the industry example above.

46 = 4/number of speeds and 6/relative torque capacity.
47 has same number of speeds as the 46 but more torque capacity, 48 same number of speeds but more torque capacity.

As for what to use, you don't mention what vehicle you are going to install the unit in. A passenger car used for normal driving would have no issues with a 42/44. It offers a lower first and second gear than the larger units, is lighter too. All of them have lockup.
Consider that the 44 was used in Jeeps and light truck, so if the unit is built correctly you should have no issues.

If you are going to flog the unit more than drive it you may want to consider the large box.