Essentially, you have FW800 (about twice the working speed of USB 2), eSATA via a Thunderbolt to eSATA adapter, USB 3.1 (5Gbps gen 1) via a Thunderbolt to USB 3.1 adapter (the only one I know of offhand happens to have an eSATA port as well), or Thunderbolt. With some higher end enclosures, ethernet may also be a possibility.
If this is a SSD and you plan to actually boot into this drive on a somewhat regular basis, USB 3, eSATA, or Thunderbolt makes the most sense (the expense here is hardware costs.) If cost is an issue, or this is a vanilla 5400 RPM HDD, or you do not plan to boot into this drive unless your main drive fails, then using USB 2.0 is the most cost efficient...FireWire 800 will have noticeably faster backup speeds, but finding well-made, readily available, and reliable enclosures at a not-completely-ridiculous price has become somewhat difficult, and investing in one of the higher end FW800 enclosures has the downside that, unless the enclosure also has USB 3.1 or eSATA, it probably won't be super useful when you replace the system.
Examples are the AKiTiO Thunderbolt to eSATA 6.0G adapter (this will take SATA SSDs up to a working speed of 550 MB/s which is essentially the SATA 6.0 limit), the Kanex USB3/eSATA Thunderbolt adapter, and the Transcend and Buffalo Thunderbolt 2.5-inch SSD/HDD drives. Oyen Digital used to make a great 2.5-inch FW800/USB3.1 enclosure for about $60, but recently discontinued it. OWC may still offer some FW800 products.