Look for vertical-grain redwood which is the most durable wood for outdoor for the spa skirt or consider an all plastic skirt for no maintenance. Check the warranty. The components that run your spa should be backed strongly by the manufacturer's warranty. Ask the dealer for a product brochure explaining in detail how each spa is made and the parts and materials used in construction. Energy efficiency saves.
Portable spas can be much more energy efficient than built-ins, but only if properly insulated.
That insulation as well as the insulating value of the cover will be the key determinants of operating costs. The most energy efficient spas use 2lb closed-cell foam. Operating costs do add up. Reputable dealers will provide documented information on typical operating costs. Heating costs for a spa with a low initial purchase price can run two or three times more a month than for well insulated models which may cost more initially.