Personally me, I'd prefer separate "daily pack".
Reason: even while relatively long walk / trekking you periodically need something (bottle of water, sunglaces, sunscreen, camera, spare battery etc) and when your small rucsac is attached to the big one you need to remove both of them to reach your things.
In the other hand you can put big rucsac on your back ("backpack") while small pack is resided on your chest. With this configuration you can reach everything you need "on place", you don't need to remove pack. Moreover in this case you are not limited with only detachable models of rucsacs :)