I prefer wider cages myself - my cage is 2 1/2 grids wide and I can't imagine making it any narrower.
And the 3x5 is bigger, as it is the floor space, ie the "area", that matters. Area is length X width.
A 2x6 cage is 12 sq grids of floor space, a 3x5 is 15 sq grids.
Imagine laying 3 of...