I looked into Amarr. www.amarr.com http://www.amarr.com/residential.html
It has many series. Heritage, Stratford, Olympus are the 3 most popular steel material ones.
For Heritage and Stratford, both have 1000( Single-Layer: Steel), 2000 (Double-Layer: Steel + Insulation ), 3000 series ( Triple-Layer: Steel + Insulation + Steel). For Olympus, it has 400( 1-3/8″ Thick | R-value 11.04
), 500 ( 2″ Thick | R-value 15.67)series. Olympus has an amazing R value 15.67, which is the index of insulation. Stratford has R-value around 6.5, while Heritage has R value of around 9.5. If you need a good insulation. Olympus or Heritage will be better than Stratford.
3 layers will be much stronger than the single or double layer ones.