It's both, depending on the segment. The vast majority of SSD controllers can easily saturate the SATA bus, so there's not really much room for differentiation

It's both, depending on the segment. The vast majority of SSD controllers can easily saturate the SATA bus, so there's not really much room for differentiation in 2.5"/M.2 SATA drives. But when you've got a PCIe-3 x4 bus to work with, as is the case for the NVMe drives being compared in this article, it's a whole different ball game. There the bottleneck is the controller (and to a much lesser extent, the flash itself). For a while Samsung was really the only game in town, as far as high-end performance goes, but it seems like WD has finally got a worthy competitor. Now it just remains to be seen how well the drives hold up (bad controller firmware can put up high performance numbers at the expense of the longevity of the flash (aka your data)).