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As a huge Magmortar fan, I can say that objectively Infernape is a much better designed pokemon, competitively speaking.
More ideal stat distribution (namely it's speed), better movepool and having a Fighting sup-type all give it a huge boon.
Magmortar's close though. If it's speed was better, and it had a nice selection of boosting moves, it could do well. As a scarfer, its great offensive coverage gives it a nice niche as a Fire-type revenge. Not complete trash, luckily.
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Design-wise, they're both very nicely done. Infernape makes a monkey look badass as fuck, and has a nice obvious homage to Sun Wukong. It's lankey body really sets it apart from other Fighting types, and gives it a cool mental appeal in terms of fighting style.
Also fire hair.
Magmortar is a cool spin on an old design. The cannons are an obvious forefront, and he just looks so badass wielding them. Not a super unique design idea, but it hasn't been done with Pokemon yet, and isn't typical of Fire pokemon to shoot fire from other places than their mouths.