With the future pointing towards 10Gbps USB 3.1 Gen2 storage using a Type-C interface, that's your best long-term bet.
Given how expensive even the cheapest Thunderbolt to PCIe or SATA SSD adapter products are (see above), USB is a more affordable choice. G-Tech ruggedized 10Gbps SATA Thunderbolt enclosure is basically the only readily available single drive Thunderbolt enclosure choice: And sadly, many cost more than the SSD you may be planning to use. At this point however, there are few single-drive ThunderBolt drive enclosures on the market. The absolute fastest way to transfer data onto a Mac SSD is using a Thunderbolt interface. You'll likely need additional USB 3.0/3.1 or ThunderBolt external drive adapter or enclosure hardware for formatting, cloning an OSX sytem image, and testing the SSD before you perform the drive swap.
Transferring or cloning your Mac's data onto a new PCIe or SATA SSD drive requires a bit of forethought before your SSD purchase.
