The insert situation: None of these covers include a pillow insert. That's standard for the category — covers and inserts are sold separately so you can choose your own fill firmness. Budget for a standard 18×18 insert alongside the cover. IKEA, Amazon Basics, and Parachute all make suitable options.
Cotton linen vs. polyester: All six covers on this list are cotton linen, which has real texture and feels different from polyester photo-print covers. Cotton linen tends to hold up better over time and doesn't have the plastic surface quality of cheaper alternatives. It's worth the slight price premium.
Aesthetic fit: The B&W covers (Mugod bridge, Mugod nightscape, HGOD building) are the most versatile — they work against any sofa color. The Beabes modern skyline and HGOD landmarks covers work better in rooms with a defined color palette. The Nicokee subway map is its own category: buy it only if you specifically want the transit reference.
Gift buying: The subway map and the HGOD landmarks cover (with Statue of Liberty) make better gifts than the single-subject designs — they communicate more about New York as a place and work for a wider range of recipients with a NYC connection.