Hi Neil, yes I would discuss it with them, but it depends very much on the terrain/surfaces you intend cycling on (e.g a lot of wet muddy tracks?) and the distances you intend to cover but for normal road and touring use I'd definitely go with 1.5. For me the RST is sufficiently comfortable with 1.5s and personally, I find sticking 1.75s on the RST a bit at odds with its sales pitch of being a fast tourer, I ditched the 1.75 at the very first opportunity, because IMO the drag was quite noticeable, and that was with just one 1.75 on the rear. If on-(normal) road isn't your primary use for it, then 1.75s might be more appropriate, but otherwise I would stick with your original intuitive choice of 1.5s.