No. There is no 'improvement' with the powered cab. Just the addition of a power amp, an on-off button and a volume knob.
Identical speaker in both. Both use Class D amplifiers, which is as 'clean' and uniform a sound as you're going to find.
Other than the fact you have 2 different cabinets, the sound they produce should be as close to identical as anything gets. It really comes down to what format (powered head or cabinet) makes the most sense for you.
One difference is that a powered head/rack has a 600w amp, whereas the powered Kab is 200w. The power amp boost function makes the powered Kab functional for monitoring on stage/practice/home etc, but the 600w powered toaster/rack will go louder, especially if you chain/stack a couple of unpowered Kabs.
... but then you could also connect two Power Kabinets.
The powered kabinet sounds better because the fact that it is "powered" sounds better than "non-powered". 
The unpowered Kabinet sounds great with Tc Bam 200 in my setup... I am also queries in direct comparison between powered and unpowered Kabinet. ?
Question:
Will the Kemper Powerhead run stereo like the unpowered?
Yes and no.
You can turn on stereo in the output menu, then you'd have left in the Monitor/Speaker output, and right in the Direct output. But only the Left/Speaker output would be powered. You'd need another amp for the Right/Direct output.
British Audio can convert it from 600w mono, to 300w per side stereo
I use a stereo amp with the unpowered toaster
