In my opinion Xie Lian is stronger than Hua Cheng.
SPOILERS FROM THE NOVEL.
The reason I think XL I stronger than HC is because
1. When XL was banished the first time and had a fight with those thirty three heavenly officials, he fought them without any weapon and spiritual power. Where as, later when HC faught with them, he was in his best state. So I think in this case even though both XL and HC had a easy win XL was deffinetly the stronger one if compared in this case.
2. XL speneded 800+ years without spiritual powers so naturally his arm strength is incomparable and after he obtained his spiritual powers he became more powerful with extraordinary arm strength that he gained in those 800 years.
3. When Bai Wuxian and HC fought, HC was indeed calm but let's be honest here, the guy will hardly show emotions even if the sky collapsed. So expecting any reaction form him is just too much (unless his dianxia is in dainger ofcourse). Anyway, in the fight BWX smashed XL's head in the stone wall for god knows how many times. But even after that XL fought with BWX (and won) after he obtained his spiritual powers from HC. And the biggest reason I think XL is stronger than HC is because if HC thought he was strong enough to beat BWX then he will not lend all the spiritual powers he had to XL. I mean there is just no way he will let XL fight such a daingerous fight not to mention XL was injured.

From IG spoilers...it show the white guy is stronger than the patch eye guy
I hope they updated more so I can see it