An interesting question this.
I don't do aikido
It's an interesting question as in the replies you can see the styles or teaching styles of the dojo where the person answering trains.
My answer, as close as I can explain it, is this:
A person who trains and attains a vast amount of Form and doesn't achieve good Flow can only use strength to try and achieve Power.
This usually results in trying to Push or Pull an attacker to the ground, when you should be Blending with his energy. Big attackers won't fall just because you look like you have nice stances as you pause in your Techniques.
A person who trains in order to achieve great Power is looking at the End of a long road and once again Pushing or Pulling like mad to get there.
You all must know the theory of, "With on eye on the goal you only have one eye left to stay on the path to get to that goal"(in my opinion you have to be crosseyed to even pull that one off
Which thirdly brings me to my choice, if I have to choice only one.
Blend with an attack, try to have good form while you are there, and let the attacker supply the Strength. You will come out with all the Power in the Universe. Or if you miss his Arm on the first strike at least you Flow out of the way, and get another chance.
Flowing is essential in Aikido. Form is also important for finishing a Technique but alone leaves you standing on the spot to get hit. Power can only be achieved be combining the other 2 properly!
A bit hard to follow?
I guess I need more practice in my Flow...heh heh.