Their effects are essentially the same in their effects, but their usage and usefulness is different..
>Reflect will be in effect for 5 turns and will halve physical damage done to any of the party Pokémon on the user's side.
Light Screen is the same, except it's Special rather than Physical.
>Iron Defense raises the user's Defense stat by two stages.
Amnesia is the same, except it's Special.
So what's the difference?
Amnesia and Iron Defense only work for the Pokemon who uses it, and wears out upon switching out. However while you remain in, it cannot wear off unless it is cancelled by moves like Clear Smog. Additionally, you can stack the boosts, so you can use Iron Defense more than once, and get a higher boost.
Reflect and Light Screen do not just affect the Pokemon who used it - it affects the whole team which makes it much more useful, as you can switch out and still have it for the next 5 turns. However it does have a limited amount of time it will work before you need to use it again, and you can't use it more than once to increase your defences again.
Anyway, Reflect + Light Screen is better - it works better for teams.