I have a Lumia 620 and I absolutely love the support offered for a budget device. Neither Apple nor Google can match it. In my opinion,
Apple-No SD slot, no fm radio,no file manager, file sharing via Bluetooth is tedious, no wireless charging, no glance screen/notification led, small screen, no clear black display, bad battery life, etc
Android-Slow, buggy, no updates at all or very few updates(cannot match MS, ever), no fm on high-end phones, no official file manager by default(Nexus) , no wireless charging, bad battery life.
Nokia Lumia 1520 absolutely kills all the competition!
A lot of the features you are mentioning here aren't available on the 620, to be precise. Such as wireless charging, notification LED, 620 had an even smaller screen than the iPhone.
And the support you are talking about, Google, or to be more specific, Motorola offers it. Apple is out of the question as it doesn't make low-end products.
But Motorola with it's Moto E and G has provided quite a lot of updates, with Android L also confirmed. The only advantage I see of a Lumia 520, 620 or above is the fact that they have pretty decent cameras compared to the low-end Android offering but other than that the Android side packs way more stuff.
I take it that your assumptions about Android being slow and laggy are because you have used a low-end device? We'll coming from a person who uses a Nexus 5 right now, I can confidentially tell you that Android isn't laggy.
We don't need an official file manager when we have a host of other pretty amazing ones.
You can't judge Android for not giving updates, while yes I admit that fragmentation is an issue do you forget that Microsoft abandoned all Windows Phone 7 devices? Windows Phone doesn't have the best history with updates, either.
I don't think that there is a huge use of FM Radio for most of us anymore, given that you can listen to it via WiFi but there are certain high end phones that still support FM, I think the HTC One M8 does as well.
We don't have bad battery life either.
If the 1520 "killed" the competition like you say it would be the best selling device right now, it isn't, it probably doesn't even make the Top 10.
I moved from Android and I'm never going back. I love my 1520 love the battery life large screen and best of all no more lags like I used to get with my Note 2.
You would be comparing nearly a two year old "SAMSUNG" device to something that launched less than an year back.
The Note 2 had weak hardware, the 1520 does not.
My phone runs Android, with specifications identical to the 1520 and everything goes along very smoothly.