The distance from the paper to the eyecup is the eye relief. However I can hold the narrower roof prism binoculars steadier than porros. That's probably because my two hands are closer and thus impart less twist to the binoculars. I also prefer roof prism binoculars when viewing nearby targets such as birds. The wider spacing of porro prism binoculars tends to show double images for nearby objects.
Center focus is a definite plus.
That might be so, but transmission differences between these and other makes are only a few percent and usually not detectable. I do find significantly visible differences in contrast when I compare binoculars in daylight. To me that's quite important. Then visit a store that has a large selection. I've found only three dealers which have a wide selection on hand.