Mesa 50/50 or matrix gt1000

Depends on your preferences really.

The Mesa will colour the sound due to it's distinct "mesa power" sound.
The Matrix will be flatter and simply provide amplification for whatever the kemper outputs.

I am sure someone can go into more details than that, but there is a basic guide

I would also add that it depends what kind of speaker cabinet you are using. If using a guitar cabinet, the mesa is probably the way to go. If you are using a PA or FRFR cab, the matrix would probably provide better results IMO.

Which approach are you going to take ...?

Also have a look at the Behringer EPQ900. I have the Matrix GT1000 and prefer the EPQ900 - I don't know why, it just feels better to me.

Check out also Mesa 20/20 and VHT 2:50:2 if you want "guitar setup".
Didnt like MatrixGT800FX with guitar cab.

Quote from pepico

Which of the two works best with kemper?

I would go with a flat neutral sounding power amp that won't color the tone.
Of those two the Matrix, or most other solid state power amps.

Go for the Matrix, or another SS power amp. All the modeling is done in the Kemper. Adding a colored, compressing tube amp to it makes things complicated and not very versatile. The Mesa will behave differently depending on how far you push it.