Is glass armor better than steel plate armor?

Yes, by quite a bit.

As long as switching from Heavy Armor to Light Armor isn't a big deal for your character.

Steel Plate is level 18 armor, nearly a dozen armors are more powerful. Glass is level 36, only Dragonscale is a more powerful Light Armor, while Dragonplate and Daedric are still higher level Heavy Armors.

If it's important you stick with Heavy Armor, better armors than Steel Plate include, from worst to best: Orcish, Ebony, Dragonplate, and Daedric. Note that actual defensive value might vary according to which armors you have the Smithing perks to double your improvement of.

Better by what metric?

In Glass's favor:

• Glass is much lighter, but with the appropriate perk, either can be weightless.

• Light armors disrupt stealth less and hamper movement less.

In Steel Plate's favor:

• Steel Plate provides only a marginally better better base armor. As such, skill levels and perks in each armor tree will dominate the relative protective value of each.

• You'll start seeing Steel Plate 18 levels sooner than Glass, I suppose that's an advantage...

• When you want to improve your Steel Plate, Corundum Ingots are 2/3 the price of what you use for Glass.

• If you invest deeply in the appropriate Armor Perks, your investment in Smithing Perks and skill levels to reach the Armor Cap would be less with Steel Plate than with Glass.

It's all about the skill tree

So given that you are waiting another 18 levels to get the same protection levels; the real question is do you prefer the stealth and movement benefits plus the possibility of Wind Walker (50% faster Stamina regen) & Deft Movement (10% chance to avoid all damage) perks of the Light Armor Tree or the earlier access to high armor levels plus the possibility of Fists of Steel (+Gauntlet Base Armor to unarmed attacks), Cushioned (1/2 falling damage), Tower of Strength (50% less stagger) & Reflect Blows (10% chance to reflect damage) perks of the Heavy Armor Tree?