HCA: Hermit Crab Association

Is there a general guideline that helps with determining if a rarely kept species is diurnal or nocturnal on advance? Do nocturnal species have any advantages that diurnal crabs lack? Two crab species I'm interested in are Discoplax magna and Perisesarma eumolpe. In my understanding the former is nocturnal and the latter diurnal.

All crabs are nocturnal, but some individuals might be awake during the day for any given reason.

Not all crabs are nocturnal, in fact a good amount are actually crepuscular.

I found them (purple pinchers) being more active during day in the warm/hot summer months, and coming out later during night in the winter.
In July and August, I often saw them running around as soon as 4pm, occasionally earlier, while in the winter, I often see none before 9 pm or even later.

(The temperatures were not too hot in the summer or too low in the winter. It was also not the case that during winter it's colder during the day and in the evening we would switch the heater on - no, quite constant temp over the day, just about 5 degrees warmer during summer. Location was Southern California.
Having pets who are nocturnal was quite nice for me as usually I stay up late being at home while working on my computer, reading and such stuff.
I write "was" as currently I'm not a hermit crab owner.)