Steel is def stronger then leather is both your heavey armor and light armor skills are the exact same. Like the poster above me said im sure you have raised your light armor skill some by being struck in combat while wearing light armor gear.
There are many sets better then the Theives guild armor from the guild iteself, such as the Nightingale gear or even the Guild Master armor you get for returning the guild to its former glory.
However light armor or heavy armor is all up to you, in the end with smithing the best gear in the game is always gear you create yourself.