EDIT：Thanks to Childe，just find out a huge mistake of mine. I have calculate the 247 EM wrong. It should only provide 41.7%， not 58.7%.
Also, I have Added the calculation of damage for using Mappa Mare.
You should use elemental mastery if you have Xingqiu and using a weapon that provide percentage ATK（Or if you use Bennett.）.
Let's do some math.
Yanfei base attack at level 90 is 240, let's use Three weapon, Favonius Codex，Memory of Dust，and Lost Prayer to the Sacred Winds to calculate damage for Yanfei's charge attack at talent level 10(without calculating scarlet, damage increase, and crits) for ATK sand and EM sand. Let's made up a sub-stats for Artifacts, 30% ATK increase, 60 EM, 50 ATK total.
Favonius Codex: 4 star weapon，provide 510 base attack on 90 level, no percentage attack increase.
ATK sand：grant 46.6% ATK increase, adding 30% from sub-stats, which will provide 76.6%750=574~575 ATK. Total attack equals 750+575+311(From Feather)+50(From sub-stats)=1686 attack. 60 EM from sub-stats equals 11.4% damage increase for Vaporize, and using 4 WoF artifact will increase Vaporize by 15%, total increase is 26.4%. Damage without calculating damage increase, critical, and defense is 16861.5(vaporize)*1.264(vaporize damage increase)*1.8822(skill)=6016~6017 damage.
EM sand： grant 187 EM，which is 247 EM total，brings us 41.7% vaporize damage increase，adding 4 WoF artifact to it equals 56.7% increase. And the ATK from percentage attack is 750*0.3=225. The finial ATK is 750+225+50+311=1336. Final damage is 1336 * 1.5 * 1.567 * 1.8822=5910～5911
Memory of Dust：5 star weapon，provides 608 base ATK，49.6 percentage attack.
ATK sand：grant 46.6 percentage attack，total up to 126.6 percentagr attack，which is 848*1.266=1073～1074 ATK. Final attack is 848+1074+50+311=2283 ATK. Base on other information from previous part，final damage is 2283 * 1.5 * 1.264 * 1.8822=8147 damage.
EM sand ：56.7% Vaporize damage increase，79.6% attack increase. 848*0.796=675 ATK，final ATK is 848+675+50+311=1884 ATK. Final damage 1884 * 1.5 * 1.567 * 1.8822=8335.
Lost Prayer to the Sacred Winds：5 star weapon, provide 608 base ATK.
ATK sand: total percent attack increase is 76.6%, which means 649~650 ATK. Final ATK is 848+650+50+311=1859. Final damage equals 1859 * 1.5 * 1.266 * 1.8822=6644~6645.
EM sand: total percent attack increase is 30%. Final attack is 848+254+50+311=1463. Final damage is 1463 * 1.5 * 1.567 * 1.8822=6472.
So from those three calculation，if your weapon have percentage ATK，you should use EM sand because the ATK is been delute. And if you have a lot ATK increase from sub-stat of artifact and no EM，you could use it too.
Then，if you use Bennett，use EM sand too. Let‘s do the calculation again on Lost Prayer to the Sacred Winds. A 10 level elemental burst bennett with Prototype Rancour will provide 764 Attack（Base Attack 756*101%）
- ATK sand damage：2623 * 1.5 * 1.266 * 1.8822=9375
- EM sand damage：2227 * 1.5 * 1.567 * 1.8822=9852
By the way, keep in mind that the damage is only for comparing EM sand and ATK sand, not for weapon compare, so don’t say that Memory of Sand is better than Lost Prayer to the Sacred Winds base on my calculations. My calculation didn’t count the damage increase(fire damage increase and other) and critical damage, those will not effect ATK and EM comparison, but will surely effect weapon comparison.
Mappa Mare: 4 star weapon, provide 565 base attack and 100 EM.
- ATK sand: 76.6% ATK, final ATK is 805+617+50+311=1783. 160 EM=28.5% increase，adding 4 WoF equals 43.5%. Final damage is 1783 * 1.5 * 1.435 * 1.8822=7224.
- EM sand：30% ATK，final ATK is 805+242+50+311=1408. 347 EM，55.3 increase plus 15% equals 70.3 increase. Final damage is 1408 * 1.5 * 1.703 * 1.8822=6770.