The room has to keep working
It is frustrating to buy a beautiful table and then turn sideways every time you pass it. Start with the route from the door to your usual seat. That everyday movement deserves as much attention as the photograph of the room.
Footprint is only the beginning
A footprint is the space furniture takes up when viewed from above. Mark the proposed width and depth with removable paper. Then include a drawer or door opening, nearby plugs and the place your feet normally rest. The listed dimensions alone do not answer those questions.
Rehearse an ordinary evening
Leave the paper outline in place while using the room. Reach for a cup, move a chair and open nearby storage. If the outline gets in the way, adjust the position or size now. Check the assembled dimensions and the delivery route as separate measurements.
Keep what makes the room easier
Choose the smaller piece if it holds what you actually need and keeps the route comfortable. A shelf can help only when its contents are reachable. The IKEA room collection is a starting point for formats, not evidence that any pictured arrangement fits your home.