Payment processors and ISOs (Independent Sales Organizations) mostly sell the same products and services. In the end, ISOs resell products and services from processors and they may build additional services on top of processors.
There are some ISOs that are huge and many are small. However, in general, they are smaller than processors. Banks are ISOs. For example, Wells Fargo is an ISO of First Data. ISOs rebrand products so it’s difficult to tell the difference between ISOs and processors.
There is a lot of flexibility for ISOs because they may sell different services from multiple processors, potentially offering a best-of-breed solution. ISOs may also technically integrate different products to innovate in payments. Many ISOs tend to be smaller and put a lot of effort to differentiate in customer support. Flexible or innovative products and good customer service is why some ISOs have become very successful.
The price between ISOs and processors is no different. It is very unlikely that price would be the reason you would choose a processor over an ISO, or vice versa.