I own a SX. I had to have 115v which was part of the reason I chose it. I'm a huge fan of the Moto-Massage and for me it would be a deal breaker vs a non Moto-Massage hot tub. I'm also not a lounger fan and I really like having a cool down seat.
You really need to see and sit in each to figure out what is right for you. I think you will notice the SX will be more spacious and will do a better job displacing water. Price will probably be similar, but I would ask what options is the Nashville coming with? Are you getting the LED light package, full foam insulation, inline sanitation?
I have a bias as an SX owner and a Hot Spring retailer. Bias aside, both are quality brands. Assuming both dealers are equal and price is similar, buy the one that fits you best.