These 2 pricing methods are very different. According to an article on merchantnegotiators.com, It is most common for credit card companies to charge their merchants by the tiered pricing model. The article further explains that this model splits all transactions into different tiers, usually 3 or 6. Each tier has a fixed amount to charge each transaction on that pier.
Interchange plus pricing, according to the article, takes the interchange rate and adds a mark up for each transaction. While the the interchange rate is nonnegotiable, the mark up amount is.
Click here to read the full article at merchantnegotiators.com, and to learn more about the difference between interchange plus pricing and tiered pricing.