All spells have several power levels. I am wondering what a different power level does for a protective spell such as Magic Screen or Armorplate? I do know that the duration is increased,but is that the only benefit from higher level power levels?

They only last longer. You don't get extra protection from higher power levels.

Armorplate rises AC by 1 at PL 1-2, for 2 at PL 3-4 ...
Each PL of Magic Screen provides 5% resistance bonus in every realm,
while the effect of Missile Shield has no reference as a number in game,
based on >100 runs through Wiz8 i think that also the effect of Missile Shield scales with PL.

btw, Enchantd Blade also scales, you can check it with the "Show Eqipment Info" button.

Townito, How did you get this information? Is it listed somewhere on the web or in the manual that you can reference for me? Thanks

Doesn't it even show that ingame? In the event log? Haven't played for a while.

If it is not shown in the game, you can get via the Cosmic Forge editor.

The info is based on comparison of the related values (or, if unknown, all character infos)
with the specific spells active at different PL, or not at all.

btw, it cant be seen in the Cosmic Forge editor, thats unnecessary for a modding tool,
i.e. it is only available in game by comparism or a statistical approach,
like also the effects of equipping items which modify attributes/skills,
or the fact that a weapon weight of >6.0 lbs will reduce a characters additional attacks/swings by one,
as long as his stats allow for bonus attacks/swings, and even more for much heavier weapons. .