I think the 1.9 is much closer to standard 700c wheel diameter, you might find the change in handling and BB drop quite significant on the 1.75
As far as I have read somewhere in 650b 1.75/42c equals 700x23c in diameter and 1.9/48c equals 700x28c.
@Prz started
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I think the 1.9 is much closer to standard 700c wheel diameter, you might find the change in handling and BB drop quite significant on the 1.75