Don't get CX9. Less rim clearance for an unnecessary small power increase.
Get CX8.4. (Or Paul mini moto if you ball hard, or RX5 if you are budget constrained)
If you get non TRP, get adjustable noodles, these are essential.
If you are getting a custom frame, get the posts set as high as possible (give the builder the brake), so you can run the pad at the very bottom, which will maximise your rim clearance / modulation.
Use compressionless housing for the best performance.
And if you're using flat bars, just go straight to MTB V brakes and levers, Shimano XT for example. All the rim clearance and vertical room you could want.
I've used Tektro Oryx (narrow), Shimano R550 (narrow), Avid Shortly Ultimate in wide & narrow, FSA SL-K (wide), Paul Touring & Neoretro (narrow & wide), MAFAC tandem (wide), Froglegg style (wide), and who knows how many others I've forgotten. plus Tektro 926AL (80mm arms), RX5 (85mm arms) minis and have CX8.4 about to be fitted.
There are certain truths:
Mini vees have more outright braking power, full stop. Even minis with 80mm arms with pads mid slot have enough power to pull endos and rip skids with one finger on drop bar levers.
The shorter the arm, the less cable pull required --> greater rim clearance possible.
Cantis have more rim clearance, but this only really matters in grassy mud or if your rims are horribly out of true.
Cantis with low profile arms and a low straddle and good pads can provide great braking, but the lever used is a factor too, eg Oryx (generally a terrible brake) works very well with a shimano BL-R400 style lever. Some of the old Deore XT medium profile cantis are the best I've ever used.
Cantis are the solution if you want to run tyres over 28mm plus fenders, or monstercross sized tyres.
Mini vees with arms 85mm or less are best.
Mini vees work better with SRAM or hidden gear cable shimano as these pull a little more cable, but they still work well with campy/ext gear cable shimano.