Earlier this year I began traveling, texting, and tweeting much more than I ever had before, resulting in draining the life of my iPhone faster than I could keep it charged. Rather than constantly keeping the iPhone plugged in (which is virtually impossible), I looked into a backup battery for my iPhone 4. There are several options on the market, which are all competitively priced. In addition to boosting the battery life, the cases offer protection against accidental drops and bumps. But how do these backup battery cases compare not only in function, but in form? Here we’ll compare the two best iPhone 4 battery cases on the market: The Energizer Energi to Go Powerskin, which I previously reviewed in March 2011, and the Mophie Juice Pack Plus, which I recently tested as an alternative.

I ultimately found that the difference between the two cases was not in function, but in the form. Both Energizer and Mophie’s cases extended the use of my iPhone by almost three days. (Note: I don’t listen to much music or watch many videos on my phone, though I am constantly tweeting, reading, and responding to email, as well as reading blogs.) The Energizer case offers up to five hours of talk time on 3G (nine hours on 2G) and up to four extra hours to browse the Internet on 3G (seven hours on Wi-Fi). The Mophie Juice Pack Plus offers eight hours of talk time (up to 16 on 2G) and seven hours of Web browsing on 3G (up to 11 on Wi-Fi). I was impressed by how well both allowed my iPhone battery to outlast those of friends and colleagues during conferences and other special occasions that can quickly drain a battery. With both of these cases, I have never needed to carry an extra USB cable or car charger with me.

The cases do, however, vary drastically in form. So what is the best iPhone 4 battery case? Here’s a look at how the two cases compare.

Mophie Juice Pack Plus (Black)

The Mophie Juice Pack Plus, which retails for $99.95, offers an ergonomically designed, fully protective, and impact-resistant case that snaps together. The material of the case is designed to be shock-absorbent, meaning that if you drop your phone, it won’t bounce like other protective rubber cases. However, the case does wrap around the phone and will prevent most accidental drops from shattering your screen. (As someone who drops their phone on a daily basis more than most people do in a lifetime, I can say from personal experience that a Mophie case will protect an iPhone from minor incidents.) The black Mophie Juice Pack Plus case is also sleek and fits snugly on an iPhone 4, with appropriate space for the iPhone power and volume buttons. The case does add noticeable bulk to the iPhone, though, causing the iPhone to weigh more than twice as much and adding nearly an inch to its height and about a half inch to the depth of the iPhone.

Mophie Juice Pack Plus (Colored)

The Mophie Juice Pack Plus is also available in a variety of colors, including yellow, purple, and magenta. I’m known for my affinity of all things pink (if you have caught a glimpse of my MacBook, you’re already aware of my obsession), so I decided to also try out the Mophie Juice Pack Plus in magenta. I was shocked to find that the design of this Mophie Juice Pack Plus was drastically different from its all-black companion. The colored sides strips of this version of the Mophie Juice Pack Plus are actually adhered to the case, which detracts from the “ergonomic” design. Mophie cases also snap together so the iPhone can slide into the case and fit snug and securely. However, with the colored version of the Mophie case, the sides did not snap together seamlessly. The design of the case was clearly different from the black Mophie Juice Pack Plus, which is not noted on the Mophie website — and a huge disappointment, as there are few personal customization options for power iPhone users.

Energizer’s Energi to Go PowerSkin Portable Case Charger for the iPhone 4

Energizer’s Energi to Go PowerSkin offers similar functionality as Mophie’s cases; both nearly double the battery life, offer indicator life to show how much charge is remaining in the backup battery case, and Energizer’s rechargeable battery charges almost as quickly as Mophie’s rechargeable lithium polymer battery. The clear difference is the design of Energizer’s case, which is a solid silicone rubber case that can fit around your iPhone 4/4S like a glove. The rubber material adds an extra layer of impact resistance. While a Mophie case will absorb the shock of a dropped iPhone, an iPhone that is dropped with an Energizer case will bounce, reducing the likelihood of shattering. Energizer’s case is also only one piece, which reduces the likelihood of destruction; Mophie’s case can snap apart with impact, leaving your iPhone defenseless. (I found that this is notably worse with the colored Mophie cases.) Though Energizer’s iPhone battery case adds even more bulk to the iPhone 4 (users will find it adds even more length to the phone, albeit less weight than Mophie cases), it is a more solid and sturdy case. For those looking for the best iPhone 4 battery case in terms of both form and function, Energizer’s Energi to Go is still the best iPhone 4 backup battery case.

Do you use a backup battery case for your iPhone 4? What is your favorite? Let us know your thoughts in the comments.