So, if you ask most people about it, they’ll say that Power weapons got a pretty bad nerf in the 6th edition ruleset. They’re probably right. Still, the new rules do offer some unique advantages that I’m going to talk about today.
First off, they give versatility. Most characters who once could buy ‘Power Swords’ can now take ‘Power Weapons’ (either changed in a FAQ or since their new codex release.) In addition, anywhere that already listed a ‘Power Weapon’ gives you the option to take any weapon you want from the four listed choices. So while the basic sword took a penalty with no gains, if you have a modelling inclination then you can increase the versatility of your space marine captains or whoever else by quite a bit.
Also, they make low strength models significantly better at killing tough things. Sure, you lose either AP or Initiative if you want a weapon that gives a strength bonus, but in exchange you get guardsman that are S5 or space marines that can wound Hive Tyrants on 4s.
Now, to talk about each weapon individually:
The Power Sword: Ah, the classic power sword. Compared to its 5th edition counterpart, (where it was AP2 regardless, as was every other power weapon,) it’s just plain worse. It gives no strength bonus, can’t kill Terminators, but at least strikes at initiative. If you are playing against armies who lack 2+ saves for whatever reason, it is a pretty good choice. Consider it the ‘All-rounder.’ It has no specialty, but can take on most things fairly well.
The Power Maul: At AP4, this hardly seems worth it at first glance… except it gives +2 strength. So, you will most definitely lose your ability to crack open big nasties, but in exchange you get extreme boosts against hordes and low toughness gribbly guys. Space Marines will cause instant death against T3 if they’ve got one of these suckers, and against lighter but still semi-armored infantry this is definitely the best choice.
The Power Axe: I’m conflicted on this guy. On the one hand, it is AP2 and cracks open any armor save, and the +1 strength means you still get a fairly good ‘To-wound’ roll. The downside, of course, is that it reduces you to I1 if you try and use it, so you’ll definitely be hitting last. Consider it a ‘Poor-mans Power Fist.’ The strength bonus isn’t as good, but it’s typically cheaper to take a power weapon than a power fist. If you know you’ll be facing lots of terminators or hive tyrants or Meganobz I’d recommend it, but against faster assault units that rely less on armor saves, it’s a waste that will get you killed. Also, if your model already has low Initiative, (For example, Crusaders can take a power weapon and are already I1,) always take this because there is no downside.
Lastly, the Power Lance. This guy is arguably the most situationally useful, but in general the Lance is pretty worthless. When you charge it gives AP3 and +1 strength, making it better than the Power sword with no real downside, but on every other turn you become AP4 without the strength bonus, making it a mediocre and less effective Power Maul. I’d say only take this if you have a squad that’s garunteed the charge and will kill whatever you attack in one turn… Though if you have a squad like that, you don’t need the benefits from the Power Lance very much, do you?
(Note: Due to bad wording on GW’s part, you could technically argue that the Power Lance gets +1 strength and AP3 on the turn that any squad charges, but that is clearly not RAI and will probably get you dreadsocked.)
Now for some Mathhammer: Here’s assuming you get 3 attacks per weapon, and how much damage you’ll cause against models with varying armor saves. (For the Power Axe I couldn’t very well display that you hit lower, so I instead reduced the number of attacks by one to represent the penalty. Keep in mind that if you’re already initiative one, the Power Axe will do significantly better than all other weapons.) (Assumes that all attacks hit and used by a S4 model. Number represents how many unsaved wounds on average.)