Dishwasher models look alike in a showroom. The differences that matter are inside the tub, in the racks, the spray system, and the filter. Start there, then check the install fit. The appliance's typical service life is on its dishwasher page.
Racks and tines
Pull out the lower rack and check its adjustability, the tine style, and whether folding tines leave room for tall pots. Check the upper rack for height adjustment and how smoothly it slides. If the store allows it, load a dish you own and see whether it fits the way you cook.
Spray system
Look at how many spray arms the model has and where they sit. Look at the filter design, because a self-cleaning filter and a manual filter ask different things of you. Check whether the spray arms come off for cleaning.
Controls and cycles
Note which cycles the model offers and whether you would use them. Extra cycles add options, and they also add settings to remember. Check whether the controls are clear to you and whether the cycle names match what you run.
Install fit and paperwork
Check the cabinet opening, the water and drain connections, the electrical needs, and the install guide. Ask how the warranty is written and what its maintenance conditions are, then read those in the paper. Ask whether the seller's paper and the manufacturer's paper are the same document.
Write the comparison down
Put the models in a grid with the same rows: racks, spray, filter, controls, fit, and paperwork. Fill each cell from the spec sheet or the unit itself, not from memory or a sales pitch.