Durability means what’s the likelihood it will tear when trying to remove the concrete. Longevity means the length of time it will last before the silicone begins to tear.
A planter that is wider and shorter will be easier to demold than a tall and narrow one.
Triple check the size that is listed in the description. The molds usually look bigger in the photos than they actually are.
Some silicone mold shops offer mostly just the standard/typical planter shape and basic home accessory designs. Others have very unique planter molds, as well as unique accessories.