CH Ignitions used to be the most popular and used, and many of them did not have auto advance. They had a separate plug in unit called "Synchro Spark" that you could buy to make a non-auto advance unit into an auto advance. They also sold units with Synchro Spark, or auto advance built in. All the RCEXL units that I know about are auto advance. I believe it is the number 4 unit that has a funky advance setup that can be problematic and is not popular at all. Most if not all the other oddball Chinese ignitions are some variation or copy of the RCEXL units, and the RCEXL units are basically a copy of the CH Ignitions. In most all cases, the parts are interchangeable. As you mentioned, the units with a metal cap must be matched to the type of spark plug you have, and that fit is critical for operation without RF interference. The CH units did not use a shielded metal cap, but instead have a shielded spark plug wire that ends in a pig tail that must be grounded to the engine. The CH and RCEXL ignitions are quite good, and I wouldn't recommend the other Chinese offerings. A complete RCEXL system can be had for a little more than 50 bucks nowadays.

The Desert Aircraft engines such as the DA-50, etc., use a completely different system for triggering the ignition, and parts are not interchangeable with the CH and RCEXL types.

There are some other units out there that I am not familiar with, but the above are the most common these days.

AV8TOR