Re: your bolded question, you would lose unlimited data. If you want to go to an unlimited data plan you could go to Sprint or T-Mobile.
Do you use your iPhone intensively? I have a feeling that if you do, you're not gonna enjoy Republic. The Defy XT isn't nearly as nice of a phone all around, and while it's unlimited, you don't get 4G.
I personally am a fan of Ting, which is a bit more like a conventional carrier than Republic, but is pay-as-you-use, and has a pretty good selection of Android devices, including the new fancy ones like a Galaxy S4. Or a $70 smartphone if you want something cheap. And lots between. If you're intense users of data/voice it will probably be more expensive than Republic though, so YMMV. For myself, with 2 smartphones and some international roaming (cause I go to Canada a lot) I spend about $50 a month.
Oh, also, if you plan to leave the US at all regularly, don't go Republic. Your phone turns into a brick outside the US if you don't have wifi. Ting on the other hand has very good int'l roaming rates.
Here's a referral link anyways:
https://zfe8hfk8g.ting.com/