Does the surprise release count as "musically"? That was a very risky move.

Y'all don't have to fight about everything. Personally I think gaga's music is more risky than bey, and no I'm not considering the eras, timeframe, or release just the music itself. And risky doesn't necessarily mean a good thing look at Artpop

Lady Gaga is more experimental than Bey. ARTPOP and Cheek To Cheek were very different from each other and all of her albums preceding those two.

Seriously I don't think they both take risks with their music.
However "Beyoncé" was a little bit experimental ; Gaga has yet to do something experimental so...

I vote for Beyoncé.

Neither

Musically - Gaga
Forms of Releasing Music - Beyonce

He announced it beforehand

C2C is a risk now? It's a risk for a classically and traditionally trained musician to release a traditional pop album with the help of a well known pop music icon?

Radiohead did a surprise release in 2007 with "In Rainbows" and again in 2011 with "The Kings of Limbs" and David Bowie did a 2013 surprise release on his Birthday for his album "The Next Day" which was months before BEYONCE, so yea there is nothing risky about a surprise release