The first, for both feasts. But genuflect on the right knee, rather than kneel.
The rubric in the 2002 Latin edition of the Roman Missal, page 739, for the Solemnity of the Annunciation is:
"Dicitur Credo. Ad verba Et incarnatus est genuflectitur. "
If the Creed were being said in Latin, the words would be:
“Et incarnatus est de Spiritu Sancto
ex Maria Virgine, et homo factus est.”
The normal rubric, that comes before “Et incarnatus” is:
“Ad verba quae sequuntur, usque ad factus est, omnes se inclinant.”
So on a normal day everyone bows from “Et incarnatus” to “factus est”. But for the Annunciation, instead, everyone genuflects, puts the right knee to the ground.
In the Roman Missal, (Catholic Book Publishing Co., New York, 1985,) page 622 the rubric for the Annunciation is:
“In the profession of faith, all genuflect at the words, and became man.”
Taken literally one would bow, then genuflect for just three words. But I think the Latin edition makes it clear that the bow is replaced by a genuflection.
From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from romanrite.com/girm.html :
“275. … b. A bow of the body, that is to say a profound bow, is made … in the Creed at the words* Et incarnatus est (by the power of the Holy Spirit . . . made man)*;”
Reference: Missale Romanum, Libreria Editrice Vaticana, 2002, ISBN: 8820972719 .