Every 'smart' phone is closed source to some degree and so, insecure. The least closed would be an Android running Replicant with only the modem closed, next would probably be a Galaxy running Cyanogenmod. iphone is pretty much a closed and controlled shop but is if not rooted then possibly more secure... just less private.

 It depends what privacy you want and what you think is the biggest threat. Personally if I had bought an iphone anyway I would trust Apple but hesitate to trust the apps.