As a rule I typically play Hardcore only except when running with folks from here. I prefer it to be honest. You don't run around with a big orange dorito over your head, making flanking a more viable proposition.
Most guns will actually work, makes your situational awareness more acute as you have find people and keep an eye on the flag indicators when playing Conquest.
DMR's are awesome in Hardcore also, as I've found out in recent times. Vehicles can't run and hide to get magically fixed, you need an Engie to fix it, you need Assaults to give you health when you've taken damage.
It's probably just me but I much prefer HC to Core when playing BF3/4.
I don't have any issues playing Core if with friends, but it's not my mode of choice given said choice. I know on the Battlefield forums there are plenty of folks that say HC is crap or even easy, I don't agree, but would like to hear from you guys as you'll no doubt give a more reasoned argument.