Whose better at using elemental effects? Maya has the action skill for it and her Cataclysm tree helps a little bit, but Gaige has the catalyst class mod and her entire middle tree is about shock/elements. Deathtrap has no elemental worth even if you can shoot him to change the element. Who do you guys think uses elements better?
Maya is actually pretty amazing at solo play if you go for the Converge skill, which does an amazing job of crowd control.
And to answer the initial question, if you spec Maya all the way to Ruin, your phaselock does every type of elemental damage, and just devastates mobs. Gaige (who, to be fair, I haven't played yet) just has a tree about shock damage, which I personally am not a big fan of, even though I know some will argue it's the best elemental.
Both normal ammo and slag ammo does 1.0x damage across the board, regardless of being flesh, armor, or shield on either difficulty. Slagged enemies, however, take 2.0x damage on top of the base elemental damage, regardless of difficulty. I.E: Shock damage to shields on TVHM is 2.5x, but slagged shields is 5.0x.
With that in mind, you should keep in mind what type of enemies you fight. Bandits and wildlife are flesh for the most part, and found more at the start of the game. The endgame has a lot more loaders and bots, so you're better off with corrosive weapons.
Shock damage is pretty neutral against both, so it's a good middle-ground DOT that completely destroys shields.
Gaige specializes in shock damage, such as Shock stormclouds when you score a critical kill, more resistance to shock attacks, a shockwave every time you reload, increased shock damage in general, Deathtrap firing out a ball of energy which you can shoot to create a giant shock explosion, and the chance to set enemies on fire in addition to the shock damage. She also has a skill that increases all elemental damage types by 6% per rank (total of 30% at rank 5) for a short time after killing an enemy.
She can also imbue DeathTrap with an elemental damage by shooting him with such. DT is already exceptionally powerful when properly specced. --- Meowahaha! Madness, Genius, the only difference between them is success! - Taokaka, Blaz Blue Continuum Shift Motoko Kusanagi is my goddess!