V-brakes are powerful, but lack modulation (i.e it's easy to lock the wheel). There are some drop-bar levers available. Apparently the Tektro is better than the Dia-Compe. Won't work with STI/ergo levers.

Cantis are more than adequately powerful but can be difficult to set-up sometimes (try setting the yoke so that the tranverse cable pulls at 90deg from the pivot-anchor line, when the brake is closed). They have better modulation thus arguably perform better than Vs on wet roads (if they have equivalent pads).

But usually you can get used to the modulation of either type.

Both types are good for loads. Both are equally durable, depends if the pivots/bushings are sealed. Some high-end Vs have a parallelogram linkage to keep the pad parallel to the rim as it travels. Those extra bushings have the potential to wear and cause a tiny bit of play.

edit: while it's true that the canti cables can get in the way of the loads with some racks, some Vs have limited fender/tire clearance because of the transverse cable.