I believe both solutions are the same at the moment.
Images are loaded in VRAM on loading/downloading of the game/application and decompressed there.
All the images of all your project are loaded there at first.
Destroying instances between a layout to another has no effect on the VRAM usage, the decompressed textures stay there.
(source: blog article about C2's image compression
Ashley said he would add a feature to load up image per layout, on start of the layout at some point (no ETA on this feature though).
When this feature will be available, than your idea of "discarding" textures to only have the current layout's textures in VRAM will be the correct way to go I believe.
In the meantime, not much to do unless keeping the number of textures of your whole project the lowest possible to stick/match with the available VRAM on the device.