It's likely that a navalized Z20 will take longer. Consider the Changes the USN made to the S70 for their naval missions. Changes to the Tail, landing gear and Rotor head. Besides if the PLAN needed a smaller ASW chopper the Z9 is based off the proven ASW platform as well as the KA27
Navalized could be either like the MH-60R (carriage wheel footprint oriented for smaller decks) or MH-60S (same carriage setup with land-based variant, big-deck oriented), given the PLAN has the need for both, they could just as well go for the latter while the R&D for the former underway.
In all likelihood, z20 project was initiated by the army, as they felt they needed a new chopper. PLAN may have looked at it but it seems they weren't interested, at least at first. Maybe they think they don't need such a helo quickly and maybe they're waiting for z20 to enter service to assess it better. Or perhaps they were hoping to wait for both z20 and z15 to be ready for production and then make a decision. Either way, it doesn't seem likely we'll see any new medium class naval helo enter service for the next several years.
I'd agree that the orders from the army will come first, as they're the launch customer, then come the air force and the navy. Z-15 however can be tricky, because:
1. the Harbin-produced version only first flight last year, by R&D cycle it won't enter serial production before 2020;
2. the "Z-15" designation has all but disappear from public domain, only its civilian designation available;
3. thus far, the only known orders / users that is of government service are either police or government flight service that provides SAR services, there has been no military application; if anything, the Airbus Helicopters doesn't have H175 listed in their military helicopter catalogue; and
4. the PLAN orders has always been rumoured, never substantiated with official account.
Thus, to this day Z-20 is still the front runner for next-gen tri-service utility helicopters, though anything beyond the baseline army aviation variant will not be forthcoming until probably 2030 or a few years earlier.
.
