Uh... they're both good, but you have to think about the Pokemon that the E4 are using.
Magmortar, the Fire type. The first E4 member will be a breeze with him, as it can just hit hard with its Fire-type STAB moves. With a powerful base 125 Sp. Atk stat, and decent Speed, it can wreak havoc. Its movepool is also large, enabling it to cover Pokemon.
He can easily wreck Aaron; however, he runs into some problems with Bertha, so it's recommended you have a water or grass-type Pokemon to take out her. If you chose empoleon, or even better, torterra, they can take care of her. For Flint, he can deal some damage with Earthquake, and probably take out Steelix, but can't hit as hard as powerful STAB on Rapidash. For Lucian, Bronzong can be taken out, as it doesn't have Heatproof, but otherwise, not much it can do. For Cynthia, Lucario and Roserade can be taken care of by him, but it has problems with Gastrodon and Milotic.
Overall, the benefit for Magmortar is its ability to kill Aaron, and get good coverage. A good E4 moveset would be E-Quake, Fire Blast/Flamethrower/fire Punch, Shadow Ball, and Ice Punch.
Electivire, the counterpart. It has problems with the first two, but can hit the last two pretty hard.
Bertha: Very Hard
So, Lucian's Pokemon have higher Sp. defense, so Electivie can breeze right through them. I's Magmortar's literal counterpart, as it has higher Atk and less Sp. atk, rather than vice versa. I wouldn't recommend him as much, as he has major problems with Bertha, more so than Magmortar, and his STAB ThunderPunch is harder to get and hits less hard than Flamethrower or Fire Blast on Magmortar.
A good e4 set would be: Earthquake, ThunderPunch, Brick Break, Ice Punch.
Overall, I'd recommend Magmortar. Honestly, though, if you use them both, they work very well together.