Chengdu - for a mainland city - has a compact feel, and due to being rather flat, is quite walkable. I'd say it's a good city for mainland rookies, because it's not so in-your-face, in spite of (naturally) having millions of residents. Public transit is good, both airport terminals have metro stations (though you'd have to transfer to get further downtown), and food is often spicy.
On the flip side, Chongqing has the most Blade Runner-esque atmosphere of any Chinese city that I've visited. It's massive, and tries to remind you of that constantly. Indeed it's hilly; however that fact does keep the usual suicidal bicyclists at bay. I'd love to return there to photograph it, highlighting monorails juxtaposed with the skyscrapers. Also, both of CKG's current terminals have metro service towards the center.
Both cities have nice day trips (including to each other), but based on your 36 hours, I'd go with Chengdu.
Last edited by BuildingMyBento; Dec 3, 2018 at 7:33 pm