What's the difference? Nothing really, they're both war mongering short-sighted militias driven by self interest.
Why is Al-Shabaab fighting Ahlu Sunnah? Because starving greedy people don't know how to share and ignorant uneducated backward people don't know how to communicate or negotiate. Their own circumstances make it difficult for them to see the bigger picture.
Btw, isn't it just silly to go to a battlefield wearing sandals?
Ahlusuna : they mind their own business.
AlShabaab : they interfere with other people's business and make it their business.
Ahlusuna : they defend their land from the invaders
Alshabaab : they rob lands that don't belong to them
Ahlusuna : don't kill innocent people intentionally, they are harmless
Alshabaab: they send innocent children to blow themselves up in the middle of large public gatherings were the majority of the crowd are innocent people.
Alshabaab : takfiiris
Ahlusuna : travel to Ethiopia occasionally to sort their differences, also they love to dhikr.
Alshabaab = satan worshippers
Ahlusuna = Allah worshippers, they might need minor adjustments and fine tuning however their intentions are pure. May Allah bless them and guide them to the right path.