Depends on workload.
If you need IOPS, then yes, the full speed enterprise drive will provide roughly 25% more (assuming 7200rpm vs 5400rpm, same number of platters, data density, and head latency). But 25% more of nothing is still nothing. You are generally going to be FAR better served by making use of SSD’s, which provide orders of magnitude more IOPs. Put your VM’s and databases on an SDD pool and backup to spinning rust.
A backup workload is a throughput workload. You won’t notice any difference in regards to throughput unless you are benchmarking. The type of pool and number of vdev’s are what matter, assuming you even have more than a 1G network (which will bottleneck you to ~125mb/s max. A single disk can easily max that out when streaming data). 10G and 40G fiber are stupid cheap to setup these days with parts from ebay.
If you are just storing data, then don’t worry about speed at all. You’re big concerns are redundancy levels (I’d never use lower than RAIDZ2), price per TB, and how many disks you want to deal with (because of power and physical space)
You can use camelcamelcamel to see historic amazon prices and get price notifications
Bestbuy, Newegg and Amazon often pull the trigger on sales very close to each other.
Reddit’s datahoarder sub also has people post what they find.