DH/LH are just part of the model names, and don’t really reflect anything that you can’t already tell about the drive.
The L/S/P/H referring to what comes at the END, such as 20A1P, 20A1L, 20A1H, 20A1S, denote interface and presence/absence of LightScribe. It goes as follows:
All series are pretty equal in their scanning abilities. The 20A1/20A4 series drives have a quirk in which most of those drives do not properly scan at 8x, with a “widely accepted” slower scanning speed of 4x, and jitter scanning of DL media will fail at the layer break [the drives cannot scan jitter on the second layer]. The 20A3 and all other newer drives seem to scan at 8x just fine, and do jitter scans on the second layer without issues.
Assuming you can find either drive model [as they are somewhat old models]*, I’d take the 20A3 over the 20A1. However, either drive is a good choice for all-around usability.
- neither series – 20A3 or 20A1 – is a recent series and both have been replaced with newer 20x and 22x series. The PATA drives of the 20A1 series were replaced with the 20A4 series. Shortly following those were the iHAxx20 drives, then the iHAxx22 drives, and any of the older DH/LH drives you may find are going to be old stock.