Hello Fai;
Yes, they ARE a bit difficult to tell apart.
This species was probably just overlooked in Hong Kong until - in 1991 - a visiting birder saw some and inspired a closer look by HK birders.
Bright-capped Cisticola has a RUFOUS supercilium, rufous sides to the neck, and rufous nape. The tail is uniformly dark with BUFFY tips, as can be seen in the photo above.
Also, Bright-capped Cisticola (also known as Bright-headed Cisticola) has a "bleating" PZEEEET call.
Here is a link to a very similar-sounding bird ( but a different race to ours) in the Philippines...
http://www.xeno-canto.org/sounds ... tLuzon09Edwards.mp3
Zitting Cisticola (formerly known as Fantail Warbler) has a whiteish supercilium and a WHITE tipped tail. See photo below. (also taken at Ping Yeung yesterday, 2010-Jan-17)