Microsoft's (MSFT -1.91%) Surface Pro 4 and Apple's (AAPL 0.49%) iPad Air 3 could both arrive in the third quarter of this year, according to recent rumors. Let's look at what we know so far about these devices, and what they could mean for the future of both tech titans.

What the iPad Air 3 might look like
Apple's seminal iPad has certainly seen better days. Sales have declined for five consecutive quarters, due to a long upgrade cycle, competition from cheaper Android tablets, and the rise of 2-in-1 devices such as the Surface. Phablets, including Apple's own iPhone 6 Plus, have also cannibalized the market for smaller tablets like the iPad Mini. Last quarter, Apple's iPad revenue declined 29% year over year, but still accounted for 9% of its top line.

Unless Apple makes a big splash with the iPad Air 3, the product line could fade away like the iPod. According to current rumors cited by Expert Reviews, the iPad Air 3 could be powered by a quad-core A8X or A9 processor, compared to the three-core A8X chip in the iPad Air 2. Like the updated Air 2, the Air 3 is expected to have 2GB of RAM and a 9.7-inch screen, but Apple might replace the entry-level 16GB model with a more practical 32GB device.

Apple's iPad Air 2. Source: Apple.

The iPad Air 3 could also be as slim as the iPhone 6 at roughly 7 millimeters. It might include an "all-in-one" USB Type-C connector from the new MacBook, but that would result in an awkward tech gap between the new iPad and the iPhone 6, which uses the Lightning port. As for new features, the Air 3 could feature "Force Touch," a pressure-sensing feature that debuted with the Apple Watch. The feature allows software to act differently based on how much pressure is exerted.

The iPad Air 3 is widely expected to arrive alongside the long-rumored iPad Pro, a 12-inch device aimed at the productivity market. Both devices should launch with iOS 9, which could arrive this fall with improvements to Siri, better app integration, and tighter connections with ecosystem plays including HomeKit and HealthKit.

What the Surface Pro 4 might look like
Microsoft's Surface line has fared better than the iPad, with sales rising on a year-over-year basis for three consecutive quarters. Last quarter, Surface revenue rose 44% to $713 million, thanks to robust demand for the Surface Pro 3. The Surface, along with other 2-in-1 Windows devices, are bridging an unfilled gap between tablets and traditional PCs. This makes them a popular way for businesses to upgrade aging PCs, since employees can use the same software in tablet and PC modes without the need for dedicated mobile apps.

Rumors suggest the Surface Pro 4 would be the largest Surface device ever, with a 12-inch screen. However, another report at PC Pro suggests a 14-inch version is also in the works, and that both devices will have a resolution of 2160 x 1440, up to 16GB or RAM, and a maximum of 1TB of internal storage. Microsoft is expected to replace Intel's (INTC 0.78%) Haswell processors with fanless Broadwell or Skylake chips. Those fanless chip designs indicate the Surface Pro 4 could be much lighter and thinner than its predecessor.

Microsoft's Surface Pro 3. Source: Microsoft.

The Surface Pro 4 is expected to be a showcase device for Windows 10, which will feature Cortana as a virtual assistant, a new Web browser, a universal app store for cross-platform apps, and deeper integration with Microsoft's growing cloud-based ecosystem. The Surface Pro 4 should also switch between tablet and desktop modes more smoothly than the jarring transition between "Metro UI" and desktop modes in Windows 8.1.

What investors should expect
Although the two devices are often compared to one each other, the iPad Air 3 and Surface Pro 4 belong to different product categories.

Research firm Gartner estimates that shipments of "traditional tablets" such as the iPad will only rise 8% annually to 233 million units this year. However, it expects shipments of "ultramobile premium" devices -- which include the Surface Pro 4 and the MacBook Air -- to soar 59% to 62 million. Research firm IDC expects shipments of Windows 2-in-1 devices such as the Surface Pro to rise 41% annually this year to 16.3 million units.

In my opinion, the rumored specs of the iPad Air 3 simply don't look different enough to convince owners of older iPads to upgrade. The iPad Pro (if it exists) might appeal to some enterprise users, but it probably won't offset sluggish sales of Mini and Air devices. The Surface Pro 4, however, could continue winning over consumers and enterprise users as a practical upgrade from aging laptops and desktops instead of tablets.