I like playing my warden, but it is fair to say we don't know what it will be like in a month, since two stances are being merged, the third is getting changed, and available gambits may get changed as well.
So, in regards to the first fellow that responded, I don't think they were trying to be rude, they are just referring to the fact that warden will be changed in a big way soon.
In answer (at least somewhat) to your initial 3 questions:
1) Guardians and Wardens can both gear and trait to be DPS; I don't know how they compare to each other, but it's quite possible they have similar DPS when aiming for it. For a player, it would be more a matter of which style of fighting they prefer, since you are playing a game after all.
If you mean damage dealt while tanking, I am not sure how much damage matters when only part of your threat is from damage.
2) To be honest, I am not sure you could notice a 4% difference since there are several random factors in damage calculation; even on one class your 'average' damage in a run can easily vary beyond 4% from another run of the same encounter. Also, I haven't parsed it in a while, but at least in the past the warden spear bleeds gave more of a damage increase than the 2% sword bonus from the racial boost. However, they are undoubtedly pretty close, so it is purely personal preference which to use. I personally like the look of a spear on my elf warden, my (also elf) champ can be the sword wielder of the family.
Oh, and not all of a guardian or warden's damage is based on the weapon DPS in any case. In answer to the original question, I don't know if guardian and warden DPS is exact enough in any case for 4% to make any difference, especially since wardens already have 2% of that from class boost, and guardians may have a racial weapon damage boost of their own.
3) Wardens currently have 4 stances, 5 if you count not having a stance set. Gambit effects are set according to which stance you are in, and no stance tends to just lack any good effects, so is not recommended. Way of the Warden is a gimp legendary stance, and matches gambit effects with one of the proper stances - I forget if it copies determination or recklessness. Determination is the tanking stance, and has the most self heals and self buffs.
The two remaining stances are both DPS stances - recklessness for melee, and assailment for ranged. Assailment seems to be pretty popular in the moors, because it's a lot easier to hit targets running around, especially with various aoe slow effects and such tossed around. The stances themselves don't increase DPS by as much as Overpower does, but make up for it with additional damage from gambit effects. However, once HD comes, recklessness and determination are being merged into a 'melee' stance, with effects based on traiting to determine whether you self-heal or do more damage. Assailment will still be around as a ranged stance, but from the dev diary it sounds like its effects are getting changed, with new debuffs added (which implies some existing effects may be taken away in trade, but they didn't say what - I won't miss the lame 2% groupwide buffs we have now though).
In short: Because of the changes coming, we don't know how 1) will be in a month, 2) There isn't much difference between the weapons so use what you want, and 3) recklessness and assailment (and maybe way of the gimp) are DPS stances now, but it will depend more on traits than stance in HD.
I hope this sufficiently answers your questions.
Gremblus, Seuer, Grinand Bearit, and various others of Arkenstone