They both hve equal base stats. See what you think by looking near the bottom this page: http://pokemondb.net/pokedex/all
Electrivire is a plain Electric Type with 1 weakness and 3 types that are not very effective and Magmorter is a plain Fire type with 3 weaknesses and 5 not very effective Types.
Here are some movesets for the 2:
Magmorter: Attack: Thunder Punch, Earthquake, Strength, Brick Break, Fire Punch, Rock Slide, Iron Tail,
Special Attack: Fire Blast, Flamethrower, Thunderbolt, Focus Blast, Lava Plume, Overheat, Psychic
Electrivire: Attack: Thunderpunch, Fire Punch, Iron Tail, Earthquake, Dig, Brick Break, Strength,
Special Attack: Flamethrower, Thunder, Shock Wave, Thunderbolt, Psychic, Focus Blast