It's a little more complex than Telstra's 4GX -- while Optus is offering a similar 700MHz service, it also has the 2,600MHz spectrum. The 2,600MHz offering works over shorter distances, but can allow increased bandwidth for data.
To muddy the water even further, Optus still offers 4G on the 1,800MHz and 2,100MHz spectra and is offering carrier aggregation on 2,300MHz. But more on that later...
Both the 5.2-inch Z3 and the 4.6-inch Z3 Compact, work on the 700MHz band. They'll also work on the 1,800MHz, 2,100MHz and 2,600MHz making them both a solid offering for the whole gamut of the Optus 4G Plus network.
Apple iPhone 6 and 6 Plus
Apple's iPhone 6 and iPhone 6 Plus will happily work across the Optus 700MHz and 2,600MHz offering -- as well as 1,800MHz and 2,100MHz, just for good measure.
Microsoft Devices Lumia 640 XL LTE
If you're after your 4G speeds on a budget, then the Lumia 640 XL LTE might be a great option. For just AU$399 you're getting a big 5.7-inch screen and a 13-megapixel camera.
Like Telstra, Optus is offering a super-fast LTE-A Carrier Aggregation connection in some areas. Whereas Telstra is using Frequency Division Duplexing, Optus uses something called Time Division Duplexing, which means it's not aggregating two different spectra, but instead using time allotments on the 2,300MHz band. Optus says this allows for a theoretical download speed of up to 220Mbps, with early tests showing peak speeds of 160Mbps.
Oppo has recently made its first carrier partnership in Australia, choosing Optus for its R7 smartphone. It's a Category 4 device, so you won' be getting the truly top speeds, but it's not exactly Gus the Snail either.
If you're after the latest Sony Xperia flagship, then Optus has you covered -- assuming you like Black. Yes, only the Z5 noir is available on contract, but that's at least nicer than it being yellow or something too colourful.
The Huawei Mate 7 is an unusual case -- while it can't work across the 700MHz spectrum, it does work on all other Optus 4G bands and supports the telco's carrier aggregation. Given that the 700MHz is such a big part of the Optus strategy, use this with care -- unless you're in a CBD (especially Canberra), where it should offer some top notch speeds.
Samsung Galaxy S6 and S6 Edge
Yes, both the S6 and the Edge will get you top notch speeds on the Optus network. Samsung is really pushing category 6 LTE into all of its flagship phone range.