The messaging packages definately cost less. I would do the messaging package, turn imessage off when you are out and about, and then use imessage when in wifi to conserve text messages. Just remember to keep your data roaming off on the phone.
Its also important to note that any incoming call that goes to voicemail while your phone is powered on will result in about 1 min of international usage, as the call has to be forwarded to your voicemail, back in the USA. I suggest either keeping your phone off or on aireplane mode when it's not actively in use, and having your outgoing message let folks know that you are out of the country and to text you instead. Incoming texts go by whatever your domestic rate is, so you can recieve all you want and only text back to the folks you need to get a hold of.