One area you might look at for comparison is the warranties. Marquis' has only one exclusion - the filter cartridge. Most other companies have many, including your water maintenance, pump seals, pillows, spa cover, ozone system, etc. These warranty exclusions certainly effect your total cost of ownership.
Marquis' cabinet (Durawood) warranty is for 7 years and includes the appearance. Most cabinets are warrantied free from defects at time of delivery. Some have longer warranties, up to 5 years, but don't cover fading, discoloring, etc.
The quality and strength of Marquis' spa cover is also exceptional. Go back and compare the covers included with the spas you are considering. Marquis spas have a three year warranty on the cover. If covered by the spa manufacturer (most will refer you to their cover vendor when you have a problem), most carry a one year warranty.