Just ordered the M7 Best, I think from here on I will only get a brush because of the handle, or if it is ManchurianI have and enjoy both. You should and probably will eventually get both... but with that said, consider this. Every wet shaver needs to try a Duke in Best, but not every shaver needs to try a Duke 3 synthetic. In fact, to my mind the M7 synthetic is much better than the Duke synthetic. The longer handle provides a better fulcrum for splaying or painting, and it keeps the hand and the lather apart. When you splay a Simpson synthetic (or any synth, I suppose) the lather gets forced up the knot onto the handle. The M7 design prevents that.
So you have a choice here between two goods- there is no 'bad' option. But while both brushes are good, only Best is best!