But how are these two titans different? And which one should you be spending your money on?
Screen and display part 1
Both phones have received praise for their bright, expansive screens: the Galaxy S6 stretches 5.1 inches corner to corner while the Nexus 6 measures a gigantic 5.96 inches.
That’s 37 percent more screen space, though the Nexus 6 navigation buttons are soft ones on the display rather than physical ones underneath as they are on the Samsung phone.
Screen and display part 2
Both phones have a 1,440 x 2,560 pixel resolution, also known as QHD (or Quad High Definition) — as the Samsung display is slightly smaller that means it’s slightly sharper too.
Another area where the handsets match is in their use of AMOLED (Active-Matrix Organic Light-Emitting Diode) technology, ensuring a sharp, colourful, high-contrast display.
Size and design
The Galaxy S6 (143mm x 71mm x 6.8mm) is smaller and thinner than the Nexus 6 (160mm x 83mm x 10.1mm). It’s also lighter (138g versus 184g) and uses more glass instead of plastic in its design.
Both phones benefit from sleek and modern designs though if you opt for the Nexus 6 you should be prepared to have to use both hands to operate it.
Internal components part 1
The internal components don’t always tell the full story in terms of a handset’s power but they’re useful to look at nevertheless.
The Samsung phone uses an own-brand Exynos 7420 64-bit octa-core CPU running at 2.1GHz and 1.5GHz; overall, that’s a step up from the Qualcomm Snapdragon 805 32-bit quad-core 2.7GHz CPU inside the Nexus 6.
Internal components part 2
Both phones have 3GB of RAM inside and although the Samsung processor has a slight edge, in reality both phones are blazingly fast and you won’t notice too much of a difference in performance.
The fact that the Samsung Galaxy S6 was released several months after the Nexus 6 is one of the reasons it has a more recent processor.
The Nexus 6 sports a 13-megapixel camera on the back and a 2-megapixel camera on the front; meanwhile, the Galaxy S6 has a 16-megapixel camera on the back and a 5-megapixel camera on the front.
Thanks to Samsung’s superior optics, low light performance and wider aperture, the S6 just about edges it in terms of the camera.
Both phones are available with 32GB or 64GB of on-board storage, but only the Galaxy S6 goes all the way up to 128GB; neither handset offers a removable MicroSD card slot.
The Samsung phone also has a heart rate sensor and fingerprint sensor, which the Nexus 6 doesn’t have, and the Galaxy S6 is available in a wider choice of colours too.
If there’s one area where the Nexus 6 really wins out, it’s in the stock version of Android, unfettered by extra bloatware and guaranteed to get updates as quickly as possible direct from Google.
Samsung’s TouchWiz take on Android isn’t necessarily bad, but it tends to be overloaded with features and its updates take longer to roll out.
Battery life and charging
Both phones offer decent battery life: though the Galaxy S6 battery is slightly smaller (2,550mAh vs 3,220 mAh) it has a smaller screen to power.
Another similarity is the fast charging technology built into the Galaxy S6 and the Nexus 6, and on top of that both these handsets support wireless charging so you can juice them up without cables.
Price and other considerations
Price is of course a major consideration, though deals on and off contract change all the time: as the Nexus 6 is older you’re likely to be able to find it for a better price.
Accessories for both phones shouldn’t be a problem as they’re both high profile handsets, but neither have a removable battery, so carrying a spare is out of the question.
The Galaxy S6 Edge
If you have some extra cash bouncing around in your pocket then the Samsung Galaxy S6 Edge is worth considering in place of the original Galaxy S6.
Apart from the higher price, it features a screen with curved edges down the sides and a slightly bigger battery, though all of the other specifications between the Samsung phones are the same.
The rest of the field
The other flagship Android phones on the market include the HTC One M9, the Sony Xperia Z4 and the LG G4, and the iPhones are always worth considering as well.
Bear in mind when these phones are going to be refreshed too: there’s expected to be a new Samsung Galaxy phone in March 2016 and a new Nexus handset in late 2015.