I have the #3 super. Here's my experience based on a few hours of testing at around 28-30 F:
The Super Spiral 2 oz heavier and can stretch more to give more room. The disadvantage is that when the entire bag is stretched, at some point, the overall loft is reduced. When the bag is stretched in one spot too far, such as the knee of a bent leg, that spot will feel cold.
Depending on the temperature, some of the extra room you gain with the extra stretch might only be usable for repositioning, not for a long-term sleeping position. The tradeoff for the flexibility is that you have to manage cold spots more carefully than with a non-stretch bag.
If you're at least fairly comfortable in a normal size bag, get the Spiral. If not, get the Super Spiral.