they are both important albums for Gaga & Yoncé. I consider them as their "signature albums".
But I vote
B'Day because:
It sold
3.3 M in USA and had the huge Radio hits
Irreplaceable with 230M Al ^^
with the iconic
"to the left, to the left"
+ None of the songs on B'Day sampled old songs (Born this way...
)
+ The Video Anthology
+ it's when they started calling Beyoncé "the Queen (Bey, of Pop/R&B, of R&B, of performances, etc....)"
REMEMBER ITS THE ERA SHE RECEIVED THE ARTIST OF EXCELLENCE AWARD the highest award given by AMAs 
It had 3 top 5 singles in US.
2 #1s in UK & 1 in US.
