It was about both and they were directly tied together. The North imposed what we would call today as sanctions against the Southern States because they wouldn't abolish slavery. The Southern states said it was their state right to decide that issue and succeeded, thus, the North went to war to preserve the Union. The reasons were complicated, and yes, did involve economics and State's rights, but slavery was the catalyst.
It was the distraction so the North could destroy the South which had become richer and more powerful.
Well the tariff was certainly objectionable to the South had was the primary reason South Carolina seceded; but I have never heard the tariff described as "sanctions". The purpose of the tariff was to protect northern industries, not to sanction the South - though the effect of the tariff certainly did hurt the South. The Yankees were pretty much indifferent to the effect the tariff had on the South.