What are the best BBM-like applications to use for a Blackberry to an Iphone?

My husband lives in Tanzania- I am in the USA, we have always used BBM to chat but recently I’ve made the switch to an Iphone. What are the best applications out there to use for a BBM like experience? I’ve heard of Pingchat, Kik, Liveprofile….anyone have any suggestions? Also, my husband has internet on his phone, so this would allow these applications to work or is Wifi needed? I’m technology challenged so I appreciate any help I can get!