3 Kannons:
Pros: Good range, can glance down some AV14 vehicles, T7 when shot at, and options for single-shot or small blast templates.
Cons: Not fearless, not great
LD even with Runtherd.
VP fodder in some scenarios (Purge the Alien, Big Guns Never Tire?).
30 Shoota Boyz:
Pros: Fearless. A tough nut to crack. Versatile (can shoot, assault, take objectives, tarpit, etc).
Cons: Cannot deal with high
AV units, probably resigned to footslogging.
More Shoota Boyz will probably help you more than a unit of Kannons in the long run. Try it both ways, see what you like!