A 60 amp breaker would be used to protect 6 gauge wire. Regardless if you set this spa to run on 40, 50, or 60 amps, use the 6 gauge. 40 amps can work on 8 gauge but you might as well put in the 6 in case you change your mind, or replace the spa in the future.
If your spa is like mine, it doesn't require a 4 wire setup. You'd need 2 "hot" leads and an insulated ground to meet code. Romex cable doesn't have an insulated ground, and therefore doesn't meet code- but it may be allowed in some areas in some installations.
And double check the wire requirements to that spa. Most, but not all Premium Sundance spas don't need the neutral- but a few do. Be sure to find the wiring requirements for your exact spa. That information can be found on Sundance's website for spas going back several years.
And also find out the specific electric requirements in your locale so you meet code.