to be fair, gaga and mariah have different field and impact. I'm pretty sure right now Gaga needs more proves to surpass Mariah's influence. Gaga is pretty new in music industry, not so many artists look up on her.
for now I voted for Mariah, her Vision of Love is acclaimed as "magna carta of melisma", so many singers looked up Mariah as their influence. The next big thing is her Christmas classic which is charting at #25 on Billboard Hot 100 right now (18 years after its release), I'm sure it will be remembered for a long time. In 1995, she introduced pop-rap collaboration (Fantasy feat ODB) and at the same year she has a longest-running song on the chart (One Sweet day) that will be written on Billboard history.
last but not least, iconic =/= impact