No, and here is why:

Illia - She is a Mage that focuses heavily upon Frost related spells.

Erandur - He uses destruction magic with one hand while welding a mace in the other, but at times he'll even use a bow.

Marcurio - Dual wield destruction magic, duel wield lighting bolt( killing 2 enemy's at once in 2-3 casts) , he has a spell to make undead,vampires,draugh flee, he casts fire spells and has frost shield

Sorry but marcurio is better, even if they cast same spells he still uses duel wield magic while the other 2 don't, plus the lighting magic has a huge range and its good vs groups

Oh and the person who is writing the skyrim wiki said once that marcurio is his personal choice.
And if i am not mistake one of those 2 can summon familiars wich is kinda too much in a dungeon than can block your way and you have to reload the game.

Erandur or Illia>Marcurio.

Illia isn't even a mage, but a Witch.

I don't like Erandur. For me, most of the companion's value has to do with personality, voice, etc. Even if they are like these, which isn't much. Marcurio actually pvssyrs quite a lot, which I prefer in a companion. If someone's going to be with me I want him/her to offer more than just packmule ability and combat backup. If I run across Illia I'll have to decide if I like her enough to try her out. If she's casting spells and Marcurio casts spells then the difference for me will come down to which one I like more.

Erandur has more powerful spells. Erandur will only use a one handed weapon when out of magicka. And Ive seen Erandur dual wield spells.

Who cares what they guy writing the wiki says?Even the strat guide says Erandur has more powerful spells that no other magic using companions have.

Have you even tried Marcurio ?
Also can you give me a link where you read that erandur has more powerful spells than marcurio?

Erandur does have a personality and says plenty of stuff. Hell he even sounds like the Dunmer Michael Caine had a baby with Bob Hoskins.

Yes, Ive tried all magic using companions with my two pure magic users (ones a 'good guy' Breton scholar/archmage, the other is a Altmer Necromancer), from Eola and Belrand, to Brelyna and Aranea.

No link for something in the strat guide, sorry.

lol! I can't stand his voice. He was very nice after we finished the quest and was so warm about offering to come with me, but all I could think about was having to listen to his voice all the time.