I don’t know about road bikes but the 140 always seems a lot of fuss when looking into the kids hardtail… it’s not like his weight needs a 160mm but 140mm means adding adapters instead of the front just bolting right on… by the time you add that I’m not sure it saves any weight…
I usually go for simplifying my life … I took off some perfectly good Avid DB5’s just because I want to keep one set of spare pads and 1 bleed kit and the other bikes are all Shimano M685/M785 (and I don’t trust the 7 yr old with DOT fluid)
I know 180/160 is popular as well but my trail bike is dual 180 as it just makes rotors interchangeable….so I need to carry less spares and can swap stuff between bikes if needed…
Weight Weenie wise (usually I only care on kids racing bike) the Ashima rotors seem pretty good so far, last set were a week from China and I think £12 each… and the previous was a Alligator Aries…. being a kids bike I stick away from any sawtooth type rotors … better not to lose 10g by losing fingers 😀 .. I don’t think you’d want them on a fat bike but they are probably fine for use on a road bike