Built-in features and the possibility of enhancing the UX and functionality by means of extensions or add-ons make these two platforms equally desirable. However, when it comes to expanding its opportunities, Magento 2 has a definite edge. From design flexibility to future-proofing your store, Magento has much more to offer.
You want to be able to customize your store to meet each of your needs, so you must ensure that the platform you choose will allow you to do so, especially if you want to sell B2B. Being open-source, Magento 2 can accommodate truly complex requirements, while BigCommerce is more of a come-as-is solution which, even though powerful, may cause you delays if a needed extension does not yet exist.