I would go with #6. I don't know what brand of tub you have or whether it is new or used. That being said there are a lot of tubs on the market now that take 60 amps, which requires #6 wire. In my case, we had a new home built and had it pre wired so we could put a hot tub on a roof deck. I didn't know much about hot tubs and didn't pay much attention to the wiring. Turns out the electrician only wired for 50 amp service. I bought a Jacuzzi tub, which will run on 50 amps, but if I have both jet pumps running, the heater will kick out. I ended up paying extra money and had the electrician run new wiring so I am now wired for 60 amps. I wished I would have wired for 60 amps in the first place as it would have saved me quite a bit of money. I say plan for the future. Pay the little bit extra it will cost you and go with the heavier gauge.