there is an asian saying that "only kids would have to choose"
with the tokyo subway system so accessible and if you dont mind the double entries
then you can easily go to both either day or night on the same day
please see discounted tokyo subway pass at
https://www.tokyometro.jp/en/ticket/travel/index.html there is no reason that you have to wait at the park entrance for re-entry (unless you time it well)
just go to nearby areas for bonus sightseeing with the subway pass in between
you can even use it to return to O Kuramae
which is 21min from ξ where Z` Rikugien
or ]Λμ΄ where μγΧμλ Higo Hosokawa Garden is
on a more serious note
if you are running short of time and HAVE to choose
Z` Rikugien is far more popular than μγΧμλ Higo Hosokawa Garden
meaning the light up event will be better funded and planned
however Z` Rikugien is worthwhile seeing under daylight also (entrance fee at 240~)
at the same time why would we want to dissuade you from
μγΧμλ Higo Hosokawa Garden especially when it is free entrance
(at least during daytime)
it only takes 16min to travel between the 2 subway stations (ξ~]Λμ΄)
where Z` Rikugien and μγΧμλ Higo Hosokawa Garden are
so it will be a piece of cake to conquer both with the subway pass day and night
have a wonderful trip ~