Richard compares the Leica DG 100-400mm f/4-6.4 Leica DG and 200mm f/2.8 Review with both 1.4x TC and 2.0x TC, with some surprising results. I have always personally felt these lenses are pretty closely matched, but the Leica DG 200mm f/2.8 is just an outstanding all-around performer. Check out Richards video and decided which is best for you.
Leica DG Vario-Elmar 100-400mm f/4-6.3: B&H Photo / Amazon / Adorama
Leica DG 200mm f/2.8: B&H Photo / Amazon / Adorama