Have riden with all 3 braking systems.
Depending on team/tandem weight, where you ride or plan to ride (flat, hilly, mountains) and if you go self contained or not, and braking technique all can make a difference.
Cantis: Have made 12 mile long descents, 7 pct grade with curving road with cantis. Anticipate your braking a bit (don't just jam the brakes as you hit the curve), alternate back and forth between front and rear brake (prevents possible over-heating but may cramp your fingers).
V-brakes are definitely a bit more responsive than cantis if properly installed/applied.
Discs: have ridden with double mechanical discs on fast descent (30+ mph) and they worked great, but consider them a bit overkill for our usage. Would avoid brake systems with fluids.
Least maintenance problems were cantis and V-brakes.
Longevity of canti brakepad record for us: 50,000 miles on a set of Scott/Matthauser pads on REAR of tandem.
Needs (real or perceived), cost and type of usage all will come into play.
If they stop you properly, what else do you need?

Pedal on TWOgether!
Rudy and Kay