Of course, it remains to be seen, but rumor has it that there will only be two models.
2. Rest of the World
It's already been determined that it'll be on all 4 of the major US carriers, so people are definitely wondering if the USA model will be able to be activated on any of them. If that's true, it's an advantage in itself, regardless of what you think about Android or Android vs. IOS.
At least in the US, people hate the idea of having to buy different devices when they switch carriers. The Nexus may give the consumer greater mobility.
Every Verizon Iphone since the 5C has had all the radio bands for full compatibility (HSPA, CDMA, LTE etc) with Verizon, ATT, and TMO---that's 3 of the 4 major carriers---sorry Sprint--AND it is UNLOCKED so it can easily be used most places in the world, so it isn't beyond the realm of possibility that the Nexus 6 could support multiple carriers with a single device---Apple has already done it and done it well.
The Nexus 6 (and most other Nexus devices IIRC) can easilly be rooted, not exactly the same as jailbreaking, but similar, without much hacking.
While Google doesn't manufacture the phone, they typically have a hand in the design and update the software themselves.
While Apple has always been pretty timely and reliable with their device updates, android devices don't always have that luxury, so knowing that there is an android device that can easily be rooted, expect timely updates and be cross compatible between carriers will offer advantages, even if some consider the dimensions less than ideal.
I would have to think that if Apple released a device that was unlocked, compatible on all major carriers and easily jailbroken, but had the dimensions of the 6+, it would still be popular since there is a sizable user base that appreciate flexibility, customization and control over their device.