All the Outlaw multi-channel amplifiers are balanced designs - they are all variations on the same basic design. I wouldn't worry too much about absolute slew rate numbers - they are all far in excess of what you would need in any program material you would ever listen to (I'm assuming you don't make a habit of listening to full power square waves for enjoyment!).

The exact input configuration, i.e. how they drive the two "sides" of the balanced amplifier is really an academic distinction. The end result is the same. Some models such as the 7900 have an additional active stage where the signal is split between the original unbalanced phase and an inverted copy to be sent to the "inverting" side of the amplifier.

Both "halves" of the power amplifier section are identical, i.e. they are both non-inverting.

You are not likely to hear any differences in sound quality between any of the Outlaw multi-channel amplifiers.