Both phones launch with Android 5.0.2 Lollipop out of the box and they carry two of the best-known Android customizations - TouchWiz and Sense. Here are a couple of videos to give you a feel for both platforms. It's quite unlikely you've never seen one, but there are plenty of improvements to both.
The Lollipop lockscreen dictates that notifications are visible and both TouchWiz and Sense follow that. They also have a clock and weather info visible, plus a number of shortcuts. Sense provides four customizable shortcuts as it has done for years, while TouchWiz offers only dialer and camera shortcuts (you can double-tap the Home key to launch the camera).
The One M9 supports double-tap to wake - tap the screen twice while it's off and it will light up. Other Motion Launch Gestures allow you swipe up to bypass the lockscreen altogether, swipe left to unlock and jump to the homescreen, right to go to BlinkFeed down to launch voice dialing.
You can also launch the camera by pressing the Volume up key while holding the phone horizontally. All of these Motion Launch Gestures work with the screen completely off.
The Galaxy S6 can use the fingerprint sensor to secure the lockscreen. You just place your thumb (or any other finger) on the key and hold for a second. The process is very resilient and works with the finger at any angle. It's quick too, there's no swiping involved (not even on the screen). There's a password fallback in case the fingerprint reading fails.
A fingerprint lock doesn't prevent someone from reading your emails off the lockscreen notification list. You can set the phone to hide the content of the notifications or not display any notifications to prevent that. You can have the secure lockscreen to be disabled based on the proximity of Trusted devices (e.g. a smartwatch or your car's Bluetooth-enabled stereo) or in Trusted locations (determined by geo-positioning).
The fingerprint sensor is also used to secure Private mode. It's an encrypted part of your storage that is only accessible when Private mode is active. You can store any type of file there, no app will be able to access it without your permission.
The sensor can also replace your username and password. Once you sign into a website (using the Internet app), the browser will offer to remember the login. Next time you just put your finger over the Home key and the phone will fill in the username and info fields and hit enter.
Fingerprints can also secure your PayPal transactions and your Galaxy Apps account.
The HTC One M9 no answer to that. You can secure the lockscreen with a password, but that's not nearly as convenient.
Sense 7 uses a tweaked Lollipop notification area. As you pull it down, it reveals the notifications, another pull brings out the quick toggles. You can long press them to jump to the relevant section of the Settings menu.
Samsung is using its custom layout with one row of quick toggles always visible, then the brightness slider, then S Finder and Quick connect buttons and then the notifications. The only option to view all quick toggles at once is to tap the Edit button.
Overall, we prefer HTC's (really Google's) design here as it's much cleaner. Having a row of quick toggles visible right away is handy, but there's too much clutter in the TouchWiz notification area.
Both TouchWiz and Sense have introduced theming in their latest iterations. Both companies back that feature up with a Theme store of their own. On both phones a theme changes the wallpaper and icon pack, plus the sound notifications. HTC themes also change the look of the on-screen buttons and the system font.
While we're on the subject, the wallpaper in the latest TouchWiz has a slight parallax effect, just like on the iPhone.
Both phones have the Lollipop-style 3D rolodex of recent apps. The HTC One M9 defaults to the Sense style 3x3 grid of apps. Unlike previous iterations there are multiple pages now so you're not limited to just 9 apps. Also, in this view you can "pin" an app so that it's always accessible.
Samsung has perfected its Multi Window feature. It's still limited to apps that support it, those are easily recognizable in the app switcher menu - their thumbnails have a shortcut in the upper right corner that opens the app on half the screen.
Alternatively, you can swipe from the top right edge of the screen and downscale the app into a floating window that you can move around.
This is another area where HTC has no alternative.
Staying in touch
An earlier version of Sense brought BlinkFeed - a homescreen pane dedicated to reading the latest news. It pulls headlines from select news sources (based on your interests) as well as showing recent stuff from your calendar and your Facebook account.
A new addition in Sense 7 learns your eating habits (the usual time you head out to lunch) and will offer suggest nearby restaurants to try.
Samsung has an alternative called Briefing, which is powered by Flipboard. Unfortunately, it doesn't tap into your social networking notifications.
The Settings screens on both phones are pretty similar, a long list of settings with a Search function to help you navigate quicker. Samsung also provides a selection of Quick settings - the ones you use the most - while HTC has put some toggles available at the top level of the menu, saving you a tap.
Winner: Samsung Galaxy S6. The fingerprint sensor plays a role in several places throughout the software - securing the lockscreen or just select files, replacing your login credentials for websites, simplifying PayPal payments. The multitasking with Multi Window feels simple and natural to use too.
The HTC One M9 beat its opponent with a cleaner notification area and better news reader, we especially liked the Motion Launch Gestures. Theming is a nice addition too, but all this is nothing the One (M8) can't do. Matching its predecessor is a damning praise for a flagship.