The maximum load capability of a shelf is partially determined by what constitutes a shelf. The EKET 4-cube model (the largest EKET) has only two approximately 13" cubicles per shelf wide. The Kallax in available in many different configurations, the most commonly used by LP collectors being the 8 (2 cubicles wide x 4 cubes tall, or 4 cubes wide x 2 cubes tall), 16 (4 x 4), and 25 (5 x 5) cube models. The 16 and 25 are the same side-to-side and top-to-bottom, and must support considerably more weight than the EKET. The 8-cubicle can be positioned upright---in which case a shelf is 2-cubes wide, or laying down---4 cubes wide. Laying the Kallax on it's side---if space allows---would seem to me to be preferable.
One may also secured an EKET to the rear wall, but it is sufficiently stiff and well-braced to not require doing so, unlike the wobbly Kallax. The Kallax is also backless---allowing dust access to the LP cover opening, and is a couple of inches deeper than I prefer, though that can be cured, as others have pointed out, with the addition of a spacer behind the LP’s. Whichever IKEA model unit one chooses, it will provide LP storage at the cheapest price per LP that I have found.