The full comparison is yet to come but I could run down some of the essentials because specifications comparisons don't matter, really.
The iPhone 5S beat Lumia 1520 in performance, both real time and in benchmarks so I'm pretty sure iPhone 6 and 6 Plus will do the same. However since both devices run most apps laglessly the speed bump may not matter all that much.
The camera lens have been improved, and they are wider f2.2(wider than Lumia 1520's f2.4), plus they have much faster focusing, etc. This would essentially make the iPhone 6 the better point and shoot to some extent because it would take photos faster, but for those that need more detail in photos Lumia 1520 would be better.
The screens seem to be on par with each other, however Lumia 1520 does have Glance but a lower PPI than the iPhone 6 Plus(won't really matter).
iPhone works with the new Apple Watch, however Lumia 1520 probably won't work with the majority of smartwatches released this year.
Moving on, iPhone 6 and 6 Plus support 1080p 60 FPS capture and 240 FPS slow-mo capture whereas Lumia 1520 supports 4K(in the Denim update.) So it depends on whether you need a lot of detail in your videos, or you need to capture motion better.
The iPhone 5S supports more LTE bands and support for LTE-A(faster speeds).
I believe I've covered the essentials.