Please keep in mind that this is not here to tell you which is better, mearly to compare the two and let everyone make an educated choice. (and, of course, tell you which I prefer

I normaly run with 10 in the attribute linked to the skill (smiting prayers/earth magic)
So first, the Pros:
Aftershock:
Large damage to low-armor targets.
Area-of-Effect.
Holy Strike:
Same damage to all targets.
Small aftercast.
5 energy cost.
Short cooldown.
Cons:
Aftershock:
10 energy cost.
Horrible aftercast.
Long cooldown.
Holy Strike:
Fairly small damage.
As you can see, aftershock has more downsides than Holy Strike, and fewer good things about it. However, the downside of Holy Strike can more than make up for this, because as a spike warrior, dps is a big deal, and aftershock makes a wonderfull final-hit spell, doing (at my attribute spread) 44 more damage than Holy Strike, 79 + 45 compared to 40 + 40. However, if you can chain knockdowns fast enough, then the 8 second cooldown on holy strike will suddenly become a great thing indeed, as you will be able to use it much more often. This is also due to it costing 1/2 as much as aftershock.
What it comes down to is this: Do you want to risk not killing the target with your spike, but saving some energy and being able to get right back to attacking, or do you want to stand a better chance of killing your target, but not being able to spike as often and not being able to follow through the spike with a strong attack? Or perhaps you want neither, and will go 16 hammer 13-14-15 strength.
My preference is Holy Strike, because the hammer swing after it usualy does about 40+ damage, and gives me one strike of adrenaline, so it makes up for aftershocks punch, and I can spike more often for less energy. I doubt I will go back to aftershock, except to continue comparisions of the two, or if my guild had a strange combo to try. Plus, with 10 smiting prayers, you can delve deeper in to the smiting skill line and see if there is anything in there to aid you. Scourge Healing anyone?

Thoughts? Comments?
-Banebow