My son is off to Paris (France) for a long weekend and wants to borrow some gear from me to 'do the sights'. Action shots won't be an issue, purely sightseeing
He is severely weight restricted with the gear so I have narrowed it down to either a 550D and IS kit lens or my G1X.
Which would you choose for IQ out of those two (I'm not a massive fan of the 18-55IS, yet it's still a dslr v a good compact)
I took my wife to Paris for Valentines day weekend a few years ago. It was strictly sightseeing/vacation time. I didn't want to be burdened down with SLR gear, nor finding myself spending too much time on shots. A simple PnS (which the G1-X is more than) that is pocketable and easy to use is more than enough to be able to capture great shots, but really to enjoy your (his) time in country.
Seven years ago I spent three weeks in Italy with just a 300D and the original Sigma 18-200. To this day I am still impressed with the IQ of the images from that trip. Personally, I'd recommend the 550D over the G1X.
Versus the kit lens, I think the G1X is the more convenient choice. I love my G1X as a city travel camera since no other compact that I know of provides its clean higher ISO. Its worked great for our travels this year. However, as is always the case, it depends on your son's photography style. The RX100 is yet more compact and may be competitive at lower ISOs. For me, if I'm not taking my 5D kit, it will be the G1X.
I think its a convenience thing and you are emphasizing traveling light. The IQ is probably nearly identical between the two. I'd think it comes down to a preference between viewfinder or LCD, focus/shutter response, and bulk/weight when doing touristy things. This sounds like more of a P&S kind of trip.
Here is the thing. once you limit yourself to a single lens, and a slow average lens at that, the only advantage of the dslr becomes the phase detect autofocus.
The lens on the G1x is superior to the kit lens, and the slight advantage in sensor iso of dslr (and it is slight) is compensated for by the faster lens. in addition, you get a significantly more pocketable (jacket pocket) camera.
However, if he did not have as big of a weight restriction, I would pick the dslr, kit lens, and a fast prime (35 f2, 50 1.4, or 40 2.8) over the g1x.
