Regarding the OP question, the peak memory use is probably identical. However each option affects when Construct 2 loads the object images.
If objects are placed in the layout, Construct 2 will pre-load all the object's images (for all animations with Sprites) at the start of the layout.
If the object is not placed on the layout and you create it at runtime, Construct 2 probably has released the object's images to save memory. This means it has to load them the moment you create the object. This can cause pauses/stuttering at runtime.
Apart from that it should be identical. Basically, any objects you use on a layout should be placed on the layout, even if you destroy them on startup (C2 will still pre-load them).
Thank you for the clarification. Now I have another question: is it possible to preload layouts, then? A lot of other games have loading screens. Suppose I am making a large game, and I want to send the player to a 'loading screen' layout while the gameplay layout is preloaded (and have some kind of loading bar sprite update on the loading screen). Would it be possible to do this? I would rather have a loading screen than have the game 'hang' while its loading a large layout.
Before I go further, is this even needed at all since GPUs don't render what's on the screen? I understand the CPU is still tracking everything that isn't rendered, and mobile CPUs aren't very powerful right now.
The alternative is to implement some sort of manual background loading and have the layout load in segments, kind of like what procrastinator suggested. However, this is a complicated issue for non-grid games and coming up with a working approach might take some time and math to do. If this is the only way, procrastinator would you be willing to help me figure something like this out? The benefits of having some method of loading large layouts would be good for everyone who uses C2.