There are flexible rubber irons which are good for riders with knee issues as they absorb some of the shock and make it easier for them to have their leg in the right position.
Irons that vary in weight make a difference as a lighter iron will mean less weight for you or your horse to carry, but will result in stirrups that fly up into the air when you drop them making them more difficult to regain as they aren't held down by weight.
Stirrups with wider tress simply give more surface area for the ball of the foot
Some irons that have rubbers are flexible that benefit the foot to help maintain the "heels down toes up" ideal position! You can get grips for the toe rest which in my opinion is really beneficial and keeps your foot in place! Irons can vary in weight and iron itself! Its really all personal choice of what helps you with your position!