Amano-Iwato
Increases ADS Ballistic damage by 10% after a crit, up to +50%.
If shooting ADS, Amano-Iwato is Chenxing's best logistics set.
Mingyi
Increases ATK by 1% for 3s after dealing electrical damage, up to +45%. Repeated hits reset the duration.
If Chenxing is not ADS and is hipfiring, then Mingyi is her best logistics set.
Amano-Iwato gives a higher max boost, and can be kept at the max of 50% indefinitely provided Chenxing doesn't leave the field, making Amano-Iwato Chenxing's highest-dps logistics squad if you're shooting ADS. If not shooting ADS then use Mingyi.
In practice they're both very close, so pick Amano-Iwato if you like to ADS, and pick Mingyi if you prefer to hipfire.
Basic Stats: +10.0% ATK
+10.6% Crit DMG Amplifier
Special Stat: +8.5% Electrical Damage
ATK and Electrical Damage for max damage.
Chenxing has decent base crit damage (60%), shoots weakspots a lot, and hits against weakspots always crit, so increasing her crit damage really helps Chenxing a lot.
Alignment Index does increase her shots damage too.
Ideally Chenxing is shooting weakspots a lot, and overal Crit Dmg Amplifier is the better stat, but Alignment Index does work well and works better if an enemy doesn't have any weakspots, although most bosses do have a weakspot and hence crit dmg is the better stat overall.
If you don't tend to shoot the enemies weakspots, or can't/won't/don't do so consistently, then Alignment Index is better than Crit DMG Amplifier, however Chenxing gains other benefits to shooting weakspots, so you will want to try shooting at weakspots whenever possible.