The iPhone 6S ships with iOS 9, which includes improvements to Siri, long awaited Apple Maps features and more, plus all the slick intuitiveness of its predecessor, iOS 8. There a few design tweaks but don't expect the overhaul we got last year, much of it looks and feels the same - just a few minor improvements around the edges.
The Galaxy S6 runs Android 5.1 overlaid with TouchWiz. Samsung's TouchWiz UI has been divisive over the years, but it's at its best here, while Android Lollipop is the cleanest and best version of Android yet.
Both OS's are seriously polished by this point, but if you favour either Google's services or Apple's then that could swing it. Obviously it also depends which you are used to as well, it can be hard to switch when you're entrenched in a particular system.
Apple does have the edge in one way though, as it's equipped the iPhone 6S with 3D Touch, which can act as a pressure sensitive shortcut to certain features or options, as essentially a light tap on certain things will do one thing, while a harder press will do another.
It's an interesting concept but when reviewing the iPhone 6S we did sometimes struggle to find a use for it. In time more developers will take the technology on board and bring it into their apps as well - at the moment it's just restricted to iOS only apps and the big hitters like Facebook.
The Samsung Galaxy S6's biggest weakness is perhaps its battery, which at 2,550mAh is smaller than the Samsung Galaxy S5's and won't typically last more than a day between charges. If you're planning to watch a film on your trip home, you can usually forget about it.
The problem here is that the iPhone 6S has exactly the same issues. If you're a heavy user, you're not going to make it through the whole day without a recharge at some point and that's not really good enough.
Both phones are held back by poor battery life but the issue is that the rest of the competition offers exactly the same thing. The only saving grace for the iPhone 6S is that iOS 9 will now share details on what's eating into your battery, but that's something Android phones have been able to do for years now.
We ran the 90 minute techradar video on both handsets, with screens on full brightness and accounts syncing over Wi-Fi in the background, to see how much life each lost in an hour and a half.
The iPhone 6S saw a decline from 100% to 70% during the test, which isn't a great result. Meanwhile the Galaxy S6 dropped just 16% in the same test - and that's with a larger, higher resolution display.
Price and availability
The Samsung Galaxy S6 can be grabbed from around £410 ($600, AU$800) for the 32GB version, while the iPhone 6S is launching for £539 ($649, AU$1,079) in its smallest size. In other words Apple's latest is quite a bit pricier, but it's also newer and it's an Apple product, so that's no surprise really.
Samsung's Galaxy S6 is quite a bit easier to get a hold of at the moment as it has been out there since March this year, most places will likely have stock on hand.
However, the iPhone 6S is still a hotly anticipated phone and is ready for launch on September 25. If you haven't already pre-ordered you're not going to get one on day one, and it'll likely take a little while for stock levels to fill up to get one from now.
Samsung and Apple have both crafted stunning phones fit for 2015 and you can't really go wrong with either of them. Samsung's is a bit older now but still packs a similar kind of punch as the iPhone 6S and will offer exactly the same high-end features as the newbie to the market.
The iPhone 6S is the best from Apple yet, but not all that much has changed from last year's iPhone 6 meaning it's not the must have device for those who upgraded last year.
If you're on a two-year contract though and you've still got an iPhone 5S or below, this is a serious upgrade and offers a lot of new features you've never had the chance to see before.
But then there's also the top of the range Android phone that no-one seems to have been able to knock off the top perch since it jumped up there in March this year. The Galaxy S6 is the best you can get but is not without fault. Poor battery life and a lack of microSD will really frustrate some Android fans but it's the best you can get right now.
The real choice between them is iOS or Android, because in most ways they seem similarly accomplished. When you really step back and take a look at the different platforms you realise both can do very similar things and it's more a case of personal taste.
If you love Android to pieces choose the Galaxy S6, if you're all over iOS be sure to get the iPhone 6S. But if you're looking for a change and something exciting, both offer exactly that - so be sure to switch over.
If you're buying a Samsung Galaxy S6 or an iPhone 6S it's very hard to go wrong.
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James is Managing Editor for Android Police. Previously, he was Senior Phones Editor for TechRadar, and he has covered smartphones and the mobile space for the best part of a decade bringing you news on all the big announcements from top manufacturers making mobile phones and other portable gadgets. James is often testing out and reviewing the latest and greatest mobile phones, smartwatches, tablets, virtual reality headsets, fitness trackers and more. He once fell over.