Considering we're not going to get a definitive answer, perhaps it'd be easier to compare and contrast a few pictures?
Seeing neither man were photographed together the only logical frame of reference is someone who was shot with both (Hulk Hogan).
(Andre is on the left and Show is on the right, of course)
While it's hard to make accurate assumptions (especially seeing camera angles and body posture could have amounted to the difference) it does seem that they're around the same height. Perhaps Show seems slightly taller across the two pics, considering Hulk comes in just below his eyes while almost leveling with Andre's.
Hogan's billed at 6'7" but his wife has stated before that he was 6'5", so it stands to reason there is no possible way Andre can be 7'5". Gagne, himself, has stated on a forum (so take this with a pinch of salt as it may not be him) that Andre was 6"11.5' and Show just above 7". Though with Shaq being slightly taller
than the Big Show at only a legitimate 7'1", those figures by "Gagne" seem to be more or less as accurate a figure as we'll ever likely to have.
I think, though, what makes Andre the more impressive big man is purely because 1) he was "the first" and 2) his sheer total size was greater than that of Big Show, as there's no denying he was the bulkier of the two by quite a bit. Though mostly due to having not undergone the surgery Show had.
Not exactly the best way to compare their sizes, but here is some images of them both standing with Hogan. Show looks massive in the nWo picture.
The issue with posed photographs is that camera tricks and props can be used to inflate the wrestler's size while in-ring photos show the real deal (compare pictures two and four in your post to see what I mean - there's no way Big Show isn't standing on something taller than Hogan for their nWo shoot).