I’ll have a camel back and saddle bag. For some reason I prefer weight on my body rather than the bike.
I have a Camelback Volt LR, it;s pretty lightly built, the fabric is almost see through. But even without the bladder it weights 520g. So probably 650g with a bladder.
A Water bottles weighs 80g, and a cage about 30g.
So it’s actually saving around a lb in weight ditching the backpack, regardless of where you put the water afterwards. I quite happily ride with a bottle in a jersey pocket if the bike doesn’t have a cage (or I’m carrying a 2nd bottle on the road bike and CBA fitting another cage).