What's the difference between the stats/damage output when I two-hand a weapon, vs when I use a Power Stance? What effects does it have on;
- Damage output?
- Stamina drain?
- Stamina damage (stagger)?
To use as an example for explanation, you can use Longswords, and my STR/DEX are equal at 30. (To avoid any unintentional ambiguation)
Best Answer
Power stance is used when you want specific moves, usually. Stamina drain is massively increased because the power attacks that you gain from the stance are pretty costly. The advantage is that you get all the advantages you would get from hitting someone with the same weapon twice, which can offer unique advantages compared to two-handing.
I have to use dual maces as an example here because it is a popular one and demonstrates what I'm talking about. Maces in Dark Souls 2 have pretty good poise damage (what you referred to as "stamina damage"), but it's not enough to stagger everything. An extremely popular "easy build" near launch was power-stanced maces because repeated application of the power attack would render even bosses completely stunlocked. This is because the power attack dishes out massive poise damage from both maces hitting.
Another possible application is status effects. Status effects have to be built up in Dark Souls, and "status effect damage" builds up indepedently of weapon damage. This means that it behooves the player to get as many hits in as short a time as possible to apply the status effect. So power stancing can be useful if you want to apply a lot of poison or bleed status damage very quickly.
As far as raw DPS goes, you are better off spamming light attack, as usual. It is the most stamina and time efficient way to deal damage if you aren't at risk of being hit. The power stance power attacks drain too much stamina to be good raw damage dealers. Especially with one-handed weapons like the mace and longsword, two-handing the weapon and repeated light attacks deal damage much more efficiently over several strikes than power stancing.
P.S.: As @user3574984 helpfully pointed out, caestuses are different. They are sort of "designed for" the power stance. Power stances offer unique moves for quite a few different weapons. If a weapon looks strange to you and you have the correct stats, try power stancing it -- it might result in something different than what you expected.