Assuming only the HDDs are drawing power and the enclosure psu has 100% efficiency we can sum up idle power useage for say a 24xLFF and 24xSFF enclosure.
Max SFF disk I know is ST5000LM000 with 5TB and draws 1.8W in active idle.
24x 1.8W = 43W, 24x 5TB = 120TB, → 43W / 120TB = 0.35W/TB
Good LFF disk I know is ST16000NM001G with 16TB and draws 6.3W in active idle.
24x 6.3W = 151.2W, 24x 16TB = 384TB, → 151.2W / 384TB = 0.39W/TB
So it seems that if you only compare hdd power draw there is not much difference.
The mentined SFF disks are almost the same €/TB as the big LFF disk here in my country.
The SFF disk are no 24/7 HDDs and does use SMR tech instead of CMR.
If you buy non 24/7 LFF disks you’ll save some money as €/TB are lower than 24/7 models.
So I would choose the LFF variant as this is the more robust technoligy.
.04 W /TB May look like a small amount, but that’s more than a 10% difference, hardly negligible. I’d still go with the LFF drives for a variety of reason mostly density and robustness.