I haven't done the 100 cm so i don't know how difficult it is.
With equivalent groups, I'd say the 100 CM one is way more difficult. Dying at VG and Gorse appears to be rather difficult and Sabetha isn't a big challenge either. And you don't even need to do Eternal in a single run.
R.I.P. Build Templates, 15.10.2019
The Eternal can be done on separate runs; you don't need to complete the entire wing in one go. That alone makes it substantially easier to acquire than Leaves No Hero Behind.
Valor Zeal [VZ] - Stormbluff Isle
100 cm is harder, but nothing outrageous. I am getting that title almost every pug run (30+ kp groups)
Definately No heroes Left Behind. Though it is doable with a good group and people have started gaining a lot of experience running CM 100.
The Eternal, while probably tougher with pug groups, is not all that difficult once you get a group of experienced people together (and can be done during different runs). The only real boss which might pose a threat is Sabatha due to people not stacking properly or so. If you die at VG or Gore you are miles away from managing NHLB.