backroad runner has the meat of it. both noises can occur within the normal range. if valve clearance is correct and mechanicals good, then any valve noise is normal valve noise. sometimes i've found that the two sounds occur under somewhat different condition, at least in the other types of engines I have more experience with.
and piston slap is mainly a matter of piston-to-cylinder clearance, which is only adjustable by changing pistons, an expensive operation. some piston clearance is necessary for the operation of the engine, so some piston slap can occur in normal operation.
valve tick would be more consistent within engine conditions- temperature, rpm, etc., and is kind of like an old-fashioned typewriter in character. piston slap would would come and go more with enginer load conditions--when running along at a constant moderate rpm, and varying the throttle from off to on-load or on-load to off throttle, the piston slap would vary distinctively, coming and going with the load conditions, while at the constant rpm the valve noise would remain about the same as valve clearance [and noise] is not influenced by engine load at a constant rpm. piston slap often has a little different sound as well, not the typewriter tickey noise, but more like a china cup in a tin can sound.
valve clearance is more a matter of temperature (valve clearance) and how hard (fast) the cam lobes are hitting the valve or rocker arm (rpm dependent), not load dependent. that's my experience anyway, even though I'm new to Subarus. hope this not TMI!