I would certainly hope the ZX2 sounds better - it is twice the price.
As I said, it sounds better to me since I compared both ZX2 and X5III for a while. Both are smooth sounding, but ZX2 adds on fun and energy if I can describe in my own words. Not everyone has the same sound preference though, thus you need to listen to compare yourself. If you have a headphone and requires bigger power to drive hard-to-drive headphones, X5III will probably be a better choice. However, I use mainly for IEMs, thus ZX2 excels and sound much better than X5III for the music I listen to.
You can't go wrong with X5III sound though, it is very good. Just that I like the synergy with my XBA-Z5, as well as other benefits of a more stable system, 3 times longer battery life of ZX2 vs X5III. If I don't have the ZX2, I would be happy with the X5III.
I am lucky to find a good buy of the ZX2 though (a local shop selling at a big discount for a sealed set), as it is roughly the same price as the X5III with a 128GB SD card. So I am comparing just by sound quality between the 2 DAPs without influence of the price.
Having through different high end DAPs with dual sabre, dual AKM, dual wolfson, etc. the DAC, whether single or dual does not matter to me anymore as they are just numbers. The highest graded DAC may not give you the best sound that you prefer (e.g. DX200 comes with 2 pieces of the top highest graded ESS Pro DAC that is much higher in cost than AKM). What I will choose for a DAP will be the sound it produces. The Calyx M that I used to have is a single Sabre DAC and produces much better sound that most of the DAPs with dual DACs anyway, but suffers the same issue of short battery life as well as unstable system.
So my own experience. Cost is not always a yardstick for sound quality, and DAC is definitely not always a yardstick as well.
X5III in this case, is a wonderful DAP well priced with very high ROE (Return of Enjoyment)!
Thus, I think there is no "Best" sound, but the most preferred sound to your ears. So trust your own ears, not your eyes.