elemental dmg weapons vs stock weapons

Im wondering, in general whats better, flame sword, lightning sword etc or simple slashing sword?

Im aware that rangers have an armor bonus vs elemental dmg, i have a firey choas axe and another axe, i was thinking of putting sundering on.

Vamp is better than sundering, just carry another weapon too.

You shouldn't be using an elemental weapon unless you're an ele secondary with conjure. Necros on your team will despise you for it as a good many of their skills greatly aid anyone who deals physical damage, and there are better weapon mods (like vamp) that you can't put on an elemental weapon.

i use an elemental weapon to bypass the warriors' extra bonus AL against physical, works out quite well~

So I should keep my fire choas axe for warriors and use my stock summit axe against rangers and the like?

Also, whats the difference between elemental dmg fire-lightning-earth, do some do more dmg or is it simply, cold blade vs fire imp is better, etc?

The elemental upgrades do nothing extra in terms of damage. They only change the type of damage you are doing.

Yes, you may bypass a warrior's extra armor against physical, but considering you WILL lose damage against a ranger, and MAY lose damage against some casters, I wouldn't recommend it. In addition, you will probably be targetting the warrior last, in which case, it doesn't matter.

A vampiric upgrade adds a constant 3 damage, possibly 5 depending on the weapon to every hit.

I know use a fiery chaos axe +22 HP and a sundering 10/10 choas axe +26 HP +10% in a stance. Works well, I do find I have my sundering axe equipped more than my Fiery.