When it goes on sale this spring, Apple (NASDAQ:AAPL) Watch will face numerous competitors: Although it will be the only wearable gadget from Apple itself, it won't be the only smartwatch available, nor will it be the only one that works with the iPhone.

In total, Apple Watch will compete with at least a dozen different devices, including dedicated fitness gadgets and Samsung's many different Gears. But of all the products likely to steal sales from Apple Watch, two stand out as the most significant.

The Moto 360 is limited to Android
The Moto 360 has been widely praised for its appearance and build quality. Motorola isn't offering anything comparable to the 18K gold Apple Watch Edition, but among more affordable gadgets, it's one of the only smart watches that can stand up to Apple's in terms of design.

The Moto 360 is powered by Android Wear, Google's (NASDAQ:GOOG) (NASDAQ:GOOGL) smart watch operating system. That gives it similar functionality to Apple Watch, including the ability to receive notifications, turn by turn navigation, and voice commands. It works with many third-party apps, and as the platform's popularity grows, many more are likely to arrive.

But Android Wear watches require Android-powered smartphones to function, significantly complicating matters. A would-be Apple Watch buyer may favor the Moto 360 over the Apple Watch, but making that purchase requires abandoning the iPhone -- a tall order. It certainly could happen, but it seems unlikely; despite its stylish looks, reviewers have found fault with the Moto 360's performance and battery life. A second- or third-generation product can (and likely will) correct these failings, but in its present state, the Moto 360 may have a difficult time winning over Android users, let alone Apple devotees.

Microsoft goes multiplatform
Microsoft (NASDAQ:MSFT) Band doesn't have that issue -- the Windows-maker's wearable gadget works with three different mobile platforms, including Apple's iOS. It's also more affordable: At $199, it's $50 less than the Moto 360 and $150 less than the cheapest version of the Apple Watch.

It sports many different sensors and syncs with Microsoft's own Health app, making it a strong alternative to Apple Watch for fitness purposes. Like Apple Watch, Microsoft Band can track its user's heart rate and steps taken. It also offers sleep and calorie tracking.

But Microsoft Band is a band -- not a watch. It has a display, and it can alert its user of incoming notifications, but the lack of a screen severely limits its functionality. With its dial, app platform, and Siri-integration, the Apple Watch offers iPhone owners much more than the Microsoft Band ever could.

Cross platform is the key difference
Because of its cross-platform compatibility, Microsoft Band is likely to be the far more menacing, direct threat to Apple Watch. Even as the most attractive Android Wear gadget, the Moto 360's dependency on a paired Android smartphone makes it a tough sell.

Of course, by the time Apple Watch does debut, it's possible there will be many more wearable gadgets on the market -- in just over a year, Samsung has released no less than five different smart bands or watches, for example, and Microsoft Band was suddenly announced and put on sale with virtually no warning.

But for the time being, there's nothing truly comparable to Apple Watch, at least for iPhone owners.