The S4Ai are more dipole/bipole designed speakers which have more of a ambient type of sound.
The SCL-3 are direct radiating and you will probably hear more detailed and higher impact type of sounds from this driver. Since it is more "direct radiating", it's not going to bounce the sound off the rear/side walls to create a more "background" type of surround. The SCL-3 is also significantly more expensive than the S4Ai.
I suppose it depends on your budget and what type of sound you want. In my experience and opinion, the surrounds don't necessarily have to match exactly with the front speakers, but they must be close. It's the front 3 speakers (left/center/right) that are critical to match as exactly as possible. You could look at some JBL Synthesis bookshelf speakers, unless you wanted something that would be mounted 'in-wall" like your original 2 choices.