To elaborate, by the way...
As I said, as I see it Theravadins and Mahayanists are just like Republicans and Democrats. It doesn't matter what they're called and it's a mistake to categorically place faith in one set of people or one set of ideas; what matters is what people actually think and do with whatever ideas or words or physical bodies they go by. You can use Theravada attachment to Vinaya in order to live in a politically unstable, malaria-infested hell-hole (under the delusion that it's some kind of holy land, like Tibet, Israel, or Mecca), half-starving yourself to death and begging for food, while saying you're on the road to peace and liberation from suffering... And you can use Mahayana notions of expedient means to indulge in sensual desire and immorality while thinking you're practicing dhamma. In the end, all that matters is discernment. And Abhidhamma and Nagarjuna were both clearly very discerning at the time. Seeing either of them in the right way could bring clarity to a person's life. That is why they were both influential, because they helped.
What it comes down to is proper interpretation. The words of Gautama were not enough; people felt they needed a further exposition. But this desire could be endless. People could quibble over how to interpret Abhidhamma and how to interpret Nagarjuna.
There are always many ways to interpret things. It's subjective or relative, but that doesn't mean it should be arbitrary. When you have a choice between how to interpret things, you always choose the interpretation which results in compassion, which results in a reduction of suffering for us all.
Thinking, "Nagarjuna was right, Abhidhamma is wrong," is one-sided and results in contentious arguing. The same goes for, "Abhidhamma was right, Nagarjuna is wrong." It could be either. It could be that they're both just words and you could use them for good or bad, if you're mindful and non-attached or if you're unmindful and attached.
I like them both.
The best things in life aren't things.