I think that I giant is much better then a hellcannon for the points you are paying.
Pro's of Giant,
Stubborn 10 has crazy attacks that can ruin an army, like yell and bawl, jump up and down, pick up and eat, all kinds of crazy stuff. Causes terror, etc etc.
Con's: Only T 5 and can be taken out easy by missle fire and magic.
But I think that in the right hands Giants can be the winning unit in a battle over and over again without any major problems.
Pro's of Hellcannon,
Unbreakable, T 6 S 6 five attacks and can shoot. Good for taking out large blocks of guys.
Cons: Which I think out weight its reliablity
Needs to take leadership every turn to do anything. If it misfires crazy things can happen, both good and bad.
Well I could keep going but what do you guys think.
Also me scooter lincoln and Kaleb rolled out a one on one fight giant vs hellcannon,
Giant one 3 to 1 and tied one for me and lincoln and for kaleb and scooter giant one 4 to 1