From the description of the Mage Armor spell:

You touch a willing creature who isn’t wearing armor, and a protective magical force surrounds it until the spell ends. The target’s base AC becomes 13 + its Dexterity modifier. The spell ends if the target dons armor or if you dismiss the spell as an action.

Compared to Studded Leather +1.
Regarding Mage Armor, …

You don’t need proficiency with any armor (instead of light armor proficiency)

If you wear armor that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can’t cast spells.

Your AC becomes “13 + Dex modifier” (equivalent)

You are not considered wearing armor (better)
This means you can benefit from traits for which you need to be “unarmored” like a Monks unarmored movement or the Magic Item Bracers of Defense.
special attacks from some creatures can destroy/weaken your armor
You need to spend a spell slot, maybe two depending on the length of your adventuring day (worse)
You only need 1 action to cast Mage Armor vs. 1 minute to don Studded Leather Armor
You would lose all benefits while in an Anti-Magic-Field while magic Studded Leather armor would still count as a mundane version of it. (Thanks, Yuuki)
Your defenses could be dispelled with Dispel Magic. This would have no effect on a magic item. (Thanks, Erik)
You do not have to carry the weight of an Armor (Thanks, Xavon_Wrentaile)