Their main differences are in R-value per inch and their strength.

• The Open-Cell is spray-applied low density SPF that weighs between 0.4 - 0.6 lbs per cubic ft when fully cured and reaches an R-value to about R-3.5 per inch. Open Cell is considered non-structural, semi-rigid/flexible SPF insulation that expands 100 - 150 times from its original volume. It also requires a vapor barrier.
• The Closed Cell is spray-applied medium density SPF that weighs between 1.5 - 2 lbs per cubic ft when fully cured and reaches an R-value to about R-7.0 per inch. Closed Cell is considered structural, rigid SPF insulation with strength about 15 - 25 PSI and expands about 30 to 50 times from its original volume.

Aside from possessing great environmental benefits, both products provide excellent air and moisture barrier in buildings as well as insulation factors.

Want to learn more about R-values?
Here's a very informative article about The Myth Of Insulation Values. It's a fascinating look at why the R-value isn't always the most important factor in determining the effectiveness of your spray foam insulation installation!