7.2K drives are slower and easier to produce, and with higher error thresholds which improves yields (and capacity). However, in terms of I/O operations each discrete disk can support, the 7.2K drives are markedly less performant than their faster brethren. Therefore they get the 'Nearline' moniker, as they'll hit I/O saturation much faster than an equivalent number of 10K or 15K disks. Therefore the storage producers need a way to convey, "we have faster stuff," so they went with MDL/Nearline.
This is how they try to encourage people who need both fast and lots of storage to go for the faster drives. Those on a budget will see that you can get (for example) a 1.5TB MDL/Nearline for half the price of a 450GB 15K drive and wonder why the upcharge. Even so, 48 7.2K RPM drives will still outperform 12 15K RPM drives. It's just that the 48 7.2K RPM drives will probably have a capacity of 30TB, where the 12 15K RPMs may only have 5TB of capacity.
Which is another way of saying...
Go 7.2K RPM when capacity is your number one goal and performance not really a goal.
Go with 15K RPM when performance is your number one goal, and capacity secondary.
Great answer by 1138 but one other aspect of note is that midline driveS usually do not have a '24/365' duty-cycle, this is often hard to spot in the specs but it means that if you drive the disks long and hard they'll fail FAR more frequently than you'd think. As an example we had a ~200 disk array full of 1TB MDLs and saw a >20% failure rate in the first year because the array was active for 20-24 hours a day, we had to swap the lot for 10Ks.
Expand your thinking beyond the "either Capacity or Performance" debate. Many of the storage manufacturers are moving toward a tiered storage solution mixing SSD, SAS, SAS-MDL/SATA. Most critical and frequently accessed data resides on the faster tiers, less accessed moved to lower tiers of storage. Result = best of both worlds for cost, capacity and performance. Great management tools that report on capacity utilization of each tier, and you grow the tier that's needed. Takes the guess work out of capacity/performance growth management. (No brands mentioned. Multiple storage providers moving in that direction and don't want to sound like an advertisement.)
Marketing.
7.2K drives are slower and easier to produce, and with higher error thresholds which improves yields (and capacity). However, in terms of I/O operations each discrete disk can support, the 7.2K drives are markedly less performant than their faster brethren. Therefore they get the 'Nearline' moniker, as they'll hit I/O saturation much faster than an equivalent number of 10K or 15K disks. Therefore the storage producers need a way to convey, "we have faster stuff," so they went with MDL/Nearline.
This is how they try to encourage people who need both fast and lots of storage to go for the faster drives. Those on a budget will see that you can get (for example) a 1.5TB MDL/Nearline for half the price of a 450GB 15K drive and wonder why the upcharge. Even so, 48 7.2K RPM drives will still outperform 12 15K RPM drives. It's just that the 48 7.2K RPM drives will probably have a capacity of 30TB, where the 12 15K RPMs may only have 5TB of capacity.
Which is another way of saying...
Go 7.2K RPM when capacity is your number one goal and performance not really a goal.
Go with 15K RPM when performance is your number one goal, and capacity secondary.
Great answer by 1138 but one other aspect of note is that midline driveS usually do not have a '24/365' duty-cycle, this is often hard to spot in the specs but it means that if you drive the disks long and hard they'll fail FAR more frequently than you'd think. As an example we had a ~200 disk array full of 1TB MDLs and saw a >20% failure rate in the first year because the array was active for 20-24 hours a day, we had to swap the lot for 10Ks.
Expand your thinking beyond the "either Capacity or Performance" debate. Many of the storage manufacturers are moving toward a tiered storage solution mixing SSD, SAS, SAS-MDL/SATA. Most critical and frequently accessed data resides on the faster tiers, less accessed moved to lower tiers of storage. Result = best of both worlds for cost, capacity and performance. Great management tools that report on capacity utilization of each tier, and you grow the tier that's needed. Takes the guess work out of capacity/performance growth management. (No brands mentioned. Multiple storage providers moving in that direction and don't want to sound like an advertisement.)