Once the phone gets updated to Marshmallow, the standby battery life will be a lot better because of Doze.
I'm also an ex WP user. Started with the Samsung and WP7 and switched after my Lumia 1020 kept freezing while being charged (and stopped charging) once 8.1 was out, and even after 3 replacement phones, the problem would still be there.
Over time I've come to appreciate a lot about Android that so far you probably haven't had time to do. Firstly, the apps that are in both WP and Android are so much better on Android, there is no comparison. Secondly, of course there are far more apps, and then things like quick charge, the speed of launching the camera and the speed with which the camera processes photos is far better than the Lumia 1020/1520.
Hopefully with all the customization mentioned above you can get past the UI issues. For me, I never felt that it looked childish. I just miss live tiles, the MS default keyboard and calendar. If you don't, In confident MS will resolve the WP10 mobile teething issues in the next month or two and then you can probably sell your S6E+ on swappa.com for about the same price as the 950XL.
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There are many other advantages. The quality of the existing apps, and rate at which they get bug fixes is also way higher than that on WP. Google Now has many advantages over Cortana, and once MS has resolved issues with Cortana for Android, you'll have both. Android flagship phones get updated once a year at a minimum, while MS has at least in the last 2 years taken 2 years to replace their flagships. Android has many more choices when it comes to fitness devices, banking apps that support fingerprint recognition and mobile check deposit, and more. That's no to say that there aren't a lot of things that are nicer in WP, but it's not just customization and apps that are advantages of Android.
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I think I may be a special case, since I really don't care about any of the things you mention. I didn't need quick charge, since I didn't need to recharge the battery twice in 24hrs (and I prefer it that way). I don't care about the camera because I own an entry-level DSLR and no cell phone in the world can come even close to that (I don't carry it with me all the time, but I also don't take pictures/videos all the time; additionally, the battery life of a phone when taking pictures or recording videos goes down really fast, and then you can't use it as a phone any more). I don't know if you can, but you should really try and compare the same photo taken with any cellphone and any DSLR. The difference is just mind boggling and I doubt you'll ever take a photo with your cellphone again.
As for Google Now, is that restricted to certain regions, since it isn't working all that well? Most of the time it just does a Google search for what I say (e.g. when I say 'what do I have on Monday' or 'show me my calendar for Monday' it actually does an image search for a picture of a calendar with Monday circled on it). I managed to get it to set an alarm and a reminder, but for everything else ('tell me a joke' finds a website with jokes) it just does a search on Google. Samsung's S Voice is just horrible and it sounds like it wants me to end its suffering.
Now, fitness devices, banking apps, fingeprint security, apps in general... I don't need any of those. I live in Serbia, so most of those aren't available, anyway. Apps are LOADED with ads, so much that I hate using them (the few ones I do use, for Reddit, Imgur, a newsreader and a couple of others maybe). Most of the apps on the store are some gimmicky pieces of software that I'd run once and forget about them. What apps do you use?
Also, since you mentioned fingerprint security, I don't think you should consider it to be all that secure. First of all, at least on the Edge+ phone, when adding a fingerprint, you also add a password 'just in case fingerprint scanning doesn't work' (so your phone is still as secure as the password you set). Scanning also doesn't work every time (about 9 times out of 10 it does) and if it can't recognize your fingerprint sometimes, I'm pretty sure it can also be fooled by someone else's fingerprint. If you read online, I think you'll find there are a lot of ways to fool the fingerprint scanner on any cellphone.
Iris scanning might be more secure than fingerprints, but I wouldn't bet a lot on it. If you want your phone to be secure, put a strong password and encrypt the content.