Definately go with the 6 gauge.
I think that 8 gauge is only good to 40 amps, but I could be wrong.
My owners manual says to only use 8 gauge copper.
You may also want to check on the length of that sealtite. I think the maximum allowable length is about 6'. You may need to run schedule 40 or schedule 80 pvc conduit and switch over to the sealtite for the final 6'. Also consider that of this, there may be a few feet of sealtite inside the spa, from the gromet to the electrical connect inside the service panel.
And also being from Minnesota, and having wired my own spa, you are allowed to do wiring on your own property. In my city, the State Board Of Electricity issues the permit, and does the inspections.
By the way, what part of the state are you from?
Brewman