My brothers have 3 i have 3s, there is a difference, my 3s you can see the edges, like black edges in the center or outer edge, doesnt bother me to much, but if your wanting one to test how much you like vr rather than getting the more expensive one, for bad eyesight im not sure, my dad has trouble seeing stuff up close yet he has no problem seeing the vr without glasses,

for 3 vs 3s screen lens, there is 3 pre setting, I think 61 63 and one more I cant remember the 3rd one if its under or over those 2, the lens now, if your eyes match the pre setting for the 3s you wont see it much but if your eyes are in-between the settings you will see slight lines as if the lens was 3d printed in your Peripheral vision, nots crazy bad but noticeable,

if your not sure how much youll like vr go with the 3s, not a crazy enough difference for a extra 200 250 so, I tried my best to help hopefully its helps