You can’t add a Sub mini with a Sub, and if you add a second Sub then both Subs need to be gen 3 or above.

Whilst you can’t dual sub a Sub Mini with a Sub using the Sonos app, you can dual Sub with a Sub (Gen3) and your existing Sub.

If you purchased a Sub 4, you would have to also replace your existing Sub to another Sub 4 or a Sub (Gen3)

See the section on Using two Subs with your home theater speaker

A dual Sub setup can even out the bass response in the room, so can be worthwhile in that aspect.

If you were just referring to a straight replacement of your existing Sub, then there really isn’t a noticeable difference between Sub generations.