they compliment each other extremely well in my opinion. Both are very analog but very different.
The Tempest can create a much broader range of sounds, its much more like starting from scratch (the way I use it) when creating sounds.
The MKIII is more dialed into a specific sound per voice. They make it easy for you to dial in a kick because the kick channel is set up to do so etc.
I like to multitrack everything so the 6 outs from the Tempest + the 8 outs from the vermona are quite a few inputs to deal with, in a good way :-)
I'm not sure If I just shared anything with you that you didnt already know

but to summarize I think these two machines are absolutely different enough to justify having both. The workflows are very different and having them play together is just too much fun.
I'm still getting my Template dialed in (in Logic) so that I can start new sessions with the two machines & just start going at it! If you use Logic and are at all interested, I can send you my template when I'm finished.