Tough question, "Which is better"
It is a matter for much debate among players, and it depends very much on the specific attack or defense situation. By specific, I mean specific variables like the race, the player, the server, how many turns you have left.
As a general answer, most players will tell you that "stacking" (having a balanced power rating over 10 even stacks of different ships) is better than "spamming" (having all your strength in your first stack or first three, or first five stacks, and then having few or single ships in the rest of the stacks as "dummies")
Many players also look down on spamming as a weak strategy, but don`t listen to them. It is very effective in certain situations.
The weakest strategy is the strategy you use over and over again, because then other players know how to attack you and defend against you.
Variety means success in this game.