Same thing really. Voice... hair colour... eye color... personality... only difference is Honoka don't have a giant syringe... but hey, never know. She don't have a nurse costume yet.
Except they're identical >_> if Honoka had longer wavy hair... they would be the exact same. I can't believe it didn't hit me sooner though. They even sound the same after playing bits of HDN MKII again. That weird high pitch whiny thing. ( Of course, referring to Japanese. )