ajohn

The easiest feature on a photo such as this is the primary projection (the length of primary visible beyond the tertials). It is relatively long on the bird shown here, indicating that this is a Eurasian Skylark (the other bird almost certainly also is Eurasian). Oriental Skylark would show virtually no primary projection beyond the tertials.

Other useful features to look for are the white trailing edge to the wing of Eurasian and the different call.

Although it used to be a regular species and even used to breed, Oriental Skylark is now a very rare bird in Hong Kong. Virtually all confirmed records in recent years have been Eurasian.

ajohn

Note that this last photo is not a Skylark, but a Dusky Thrush. Skylarks would never perch in trees in this way, and also the markings (white supercilium, black mottling on underparts, etc.) is more well marked than a skylark.
It has been a good winter for Dusky Thrush, and it is perhaps more likely that this is the species you've been seeing at Nam Sang Wai, Middle Gap, etc.