Should I use a GFCI breaker or receptacles for my shed run?
Because if an appliance trips the GFCI breaker, there go the lights
The issue with having your whole shed on a GFCI consists of two parts:

some things (power tools, hot things) can pose a stored-energy hazard after shutdown for a little while
tripping the GFCI breaker will plunge you into the dark as the lights will have to be on it as well

Being in the dark around a stored energy hazard, such as a spinning saw blade, is bad, so we don't want the lights to go out if that device malfunctions and trips the GFCI.  Furthermore, with the GFCI tucked inside the shed, that keeps it reasonably protected from the weather; this isn't an issue for most folks, but can be if your breaker panel is outside (say, on a meter-pole).
Besides, the difference between 12&quot; and 18&quot; burial depth isn't significant in terms of trenching costs as far as I know; if you really care that much about trench depth, you're better off using RMC and matching non-threaded fittings, as that gets you down to 6&quot; of cover anyway.