Well it's all personal preference really. Do you want to have to carry around an extra cable+battery pack, or just carry about your bulkier phone.
If you choose a case, there's cheaper/higher powered alternatives to the Mophie as well. Anker makes one for almost half the price
Thanks all. I ended up buying a new Mophie Juice Pack Plus on Ebay (verified that it was 100% authentic before buying) for $70. Hard to turn down that deal. Based on all the reviews I read, Mophie seems like the most trusted brand.