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).