Here we go with some EU3 stats guys !
BIG SHIP :
Early game = Hull : 30; Cannons : 60 and speed : 4
Late game = Hull : 60; Cannons : 120 and speed : 5
Base price : 50 ducats
GALLEY :
Early game = Hull : 14; Cannons : 15 ( +1 damage in sea, -2 in oceans ) and speed : 8
Late game = Hull : 20; Cannons : 35 ( '' ) and speed : 8
Base price : 10 ducats !
AND, with naval tech, the shock modifier is high at start of the game for galley : +1 per level of tech, when it's only +0.5 for big ships.
Then, after naval tech level 27, shock modifier become higher for big ships +1.1; +1.2 ... until +3 when it's always +1 for galley ( same for fire modifier )
So it's pretty clear that the difference beetween them is " small " at start of the game and will increase with the time !
Moreover, the base price is very high for big ships, so it's gonna be hard to create and maintain them at start, when economy is bad; and same thing, it's gonna be easier with time !
So, like in history, the galleys will disappear with the time cause they just become obsolete !
But, it's pretty clear too that some nations which are access on land can't maintain a big ship fleet until they have a big economy ! So, galleys are the solutions for them ( if they are in sea )
For example : Ottomans, Russia, Sweden ( if focusing on land ) !
Finally, let's back to the subject : I think that a nation like Venice would not maintain a 100% galley fleet until the end of the game.
This country is focusing on naval, so it can be possible and recommended to build some big ships too, and in the long term only them !