I had this dilemma this morning and went for GIT.
Aventus is great on any day to be honest, it gives me a warm upbeat feeling and makes me feel confident.
I went for GIT though because aside to being new in my collection the 'fresh' notes, especially that violet note, seem to be very well suited to wet weather and the whole idea of the 'Irish countryside' being associated with constant rain made the choice for me.
Both are great allround scents for me, but somehow I feel that GIT is at its best on a rainy day.
True! Besides I sense an incense vibe on Aventus, that for some particular reason does not seem to suit a rainy day, in another hand GIT has some sort of natural "wetness" in itself that just integrates better with the rain (rainy day).