Main difference is going to be a longer warranty on the Signature vs. E-series.
The insulation that comes on the 660 is a heavy duty foam on the back side of the cabinet, and an icynene soft spray foam over the plumbing. If you want, you can upgrade to a full foam insulation for not too much. The standard model insulation without the full foam will be energy efficient. Tests show that you save $2-5 a month with the entire cavity filled with foam.
$9 a month seems very low, they must be basing that off of a super cheap KWH energy cost. I don't know what the energy cost is in Canada, but the 660 national average in the US is around $20-25 per month.
A lot of things are optional on the E-series vs being standard on the Signature line. You have the option to add an ozone system, inline SpaFrog maintenance system, full foam, etc to the E-series.
There are some higher volume jets in the Signature series, but the pump output will be very similar - 320 gallons per minute on Signature vs 290 gallons per minute on E-Series.
There are some other small things too, like the control panel on Signature series being a full color display, and the E-series is a smaller display. Some of the Signature models also have secondary control panels, E-series doesn't. You can get an optional Wi-Fi transceiver on Signature and E-series that allows you to control the spa with the Marquis app on an iPhone.
Overall, the 660 gives you a lot of performance and quality for the price.