Paper money's days might be numbered. Credit cards started the $5 bill's long march to oblivion, and these days PayPal, Venmo, and crypto-currencies like Bitcoin are hastening its demise. Later this year, Coin--an electronic device that's the same size and shape as a credit card--promises to let you swipe your way around at retail, while offering security and flexibility through a Bluetooth connection to your phone.

But before Coin arrives on the scene, several smartphone cases already provide build in the ability to perform mobile transactions alongside protecting your mobile device--I spent some time with two of them in advance of Coin's impending arrival. Think of these accessories as phone caes that double as your credit card collection.

Incipio Cashwrap

The $50 Cashwrap is one of several iPhone cases that work with the Isis Mobile Wallet standard. In a nutshell, Isis--which, in light of current events, is in the process of rebranding itself--lets your phone case behave like a contactless, no-swipe credit card. Anywhere you see the contactless symbol on a point of sale credit card terminal, you can tap your phone to make a charge.

The Cashwrap is designed for the iPhone 5 and 5s and has an embedded microNFC chip that taps into the ISIS Wallet app (available for AT&T and Verizon subscribers) to securely stores your credit card data. Unfortunately, ISIS is far from a comprehensive solution. It works with just a handful of select cards from Chase, Wells Fargo, and American Express. To load a participating credit card into your phone, you complete an online form that adds it to your ISIS account.

But what if you already own a wallet full of non-compatible credit cards, like Discover or Bank of America? That's where the American Express Serve card comes in. Serve is an Isis-compatible debit card you can charge up with cash from any ordinary credit card. (You even get a $15 incentive for signing up.)

To make a purchase, you launch the app (entering your 4-digit PIN), and place the phone near the wireless symbol on a compatible point of sale terminal. Assuming you have more than one card in the Cashwrap, using the desired card at checkout is a simple matter of swiping among the options as if you were selecting a Pokemon card.

When the opportunity presents itself, Isis works great. Unlike with Loop's ChargeCase--which we'll talk about in a moment--you won't have any anxiety at the moment of purchase, wondering if the terminal will work with your newfangled charge case. If you see the contactless symbol on the terminal, the Cashwrap simply works, every time. The trade-off, though, is that the opportunities to use ISIS are far smaller. Over a week of shopping and routine errands, I found that only about one in five stores supported the contactless charging required by ISIS.

Moreover, the Cashwrap case doesn't do double duty as a battery, so it's a lot of bulk to add to your iPhone just to make occasional purchases. Which meant that I frequently found myself swapping iPhone cases, and sometimes found myself at the store without the Cashwrap when I really wanted it. And since ISIS doesn't work everywhere, you'll need credit cards with you anyway.

Bottom line: at this point, it's really hard to justify adding an Isis case like the Cashwrap to your iPhone. That said, if you have an Android phone with integrated NFC, the news is much better--you can use the ISIS Android app (available for T-Mobile, AT&T, and Verizon subscribers) without adding a case.

Loop ChargeCase

Loop's $99 ChargeCase, available for the iPhone 5 and 5s, doesn't rely on the same wireless transaction technology as Isis. Instead, it generates a magnetic field that mimics the stripe on the back of your credit card. Hold the case next to a sales terminal to simulate swiping your actual credit card. It's genius, because that means--in principle, anyway--ChargeCase should work almost all the time.

And indeed, ChargeCase is far more likely to work for your routine purchases than the Cashwrap. During a week of routine shopping, ChargeCase was able to complete my transactions about 75 percent of the time. I couldn't get it to work at gas pumps or ATMs, and there was some social resistance as well. At one particular coffee shop where the cashier needed to swipe the card behind the counter, no amount of persuasion could get her to take my phone and hold it near the swipe bar: she insisted her terminal didn't work with contactless credit cards and wouldn't humor me.

Thus, there was always a certain amount of anxiety using the ChargeCase. At any given store, it was never clear where to position the phone, or if it would work at all with that particular kind of terminal. (Sometimes I simply couldn't get it to work, gave up, and pulled an old fashioned credit card out of my wallet.) This generally took place as a line of unamused shoppers formed behind me.

If the case sometimes fails as a virtual wallet, at least it does double duty as a battery. ChargeCase can add about 60 percent charge to your phone, which, unlike the Cashwrap, makes it worthwhile leaving attached all the time. But the case has an unfortunately slick finish; if it were carved out of a block of wet ice, the ChargeCase wouldn't be appreciably more slippery.

ChargeCase comes with a small Square-like credit card dongle that you'll only need when loading your card information into the accompanying app. When it comes time to make a purchase, you launch the app, enter a PIN, and choose the card you want to use--or just "swipe" with the default card. Interestingly, you can trigger the swipe signal by pressing a button on the case, which you can enable to work all the time, or only within a certain timeframe of having entered the passcode on the app. You can trade convenience for security by leaving it active all the time, or only within a few minutes or hours of the last time you entered the passcode.

Between the integrated battery, broader credit card support, and a better overall success rate with making transactions, ChargeCase is the smarter solution overall, though it only works with the iPhone, and the fat, slippery case seems like it's begging for an eventual catastrophic drop. If you really want to get on board smartphone-based credit card purchases, ChargeCase is the slightly better option as of this writing. But with Coin just around the corner, it might make even more sense to just wait.