Typically, I rather prefer rock and indie music, such as Weezer, Smashing Pumpkins, Rise Against etc., but to test my headphones, some calmer tracks are often better... My all-time favorite (to test and in general): 9 Crimes from Damien Rice; with good cans, you can hear the little “moaning” in Lisa Hannigan’s voice in the chorus.
Regarding the differences, I would say that the Sennheiser’s sound fuller, and have perhaps a greater “musicality”, while the Hifiman’s are probzbly more analytical. Anyway, both are great cans, you are not going to be disappointed with neither of them.
Three last remarks:
1) Wearing comfort is considerably better with the Sennheisers, the Hifiman are quite heavy and don’t fit as well over a longer listening period.
2) You will neec a headphone amp to deive the Hifimans, they are simply not loud enough on a simple source; the Sennheisers are ok without (but obviously sound much better with an amp...).
3) After all, I have to say that neither of these two headphones is my favorite one... The one that simply makes everything right, is precise, with incredible detail and simply sounding great for endles, fatigue-free listening sessions is my faithful Beyerdynamic DT880 (150 Ohms) - similar price, so for sure an alternative to consider...
Hope that helps, good luck chosing the right headphone!