Eyecup design won’t interest you unless you wear glasses. Eyecups shield your eyes from the surrounding light so that you can get a better view down the barrels of the binoculars. Eyecups will interfere with eyeglass wearers as they extend the distance between your glasses and the eyepieces.
Flexible eyecups can be adjusted by folding the rubberized cups back and out of the way. Your eyeglasses can be set right onto the eyepieces of the binoculars if you like. Other eyecups that twist up or pop out might be preferred as they can be adjusted very quickly and require only one hand to do so.
If you wear glasses or sunglasses while using binoculars, you’ll want to see the eyecup design and get a feel for how they can be adjusted. After you handle them you’ll know which eyecup design you prefer.