The UI on the Galaxy S6 edge is a lot better than previous iterations. Virtually no lag.
Seriously, the iPhone 6 lags too. My brother has it and I've seen it lag and freeze before.
It is an upgrade over the iPhone 6, technologically. The camera is a huge upgrade IMO because you don't get OIS on the iPhone 6 and don't get the DSLR-like capabilities that the Galaxy S6 / S6 edge has.
The dual curved display technology is something you will never find with any other brand of phone. Maybe in the future, Apple may opt to get curved displays from Samsung. But it's not happening in 2015, surely. Maybe 2016.
Another upgrade over the iPhone 6 are the 14nm processor, DDR4 RAM and UFS flash memory. All speed tests confirm that the Galaxy S6 is about 50%~250% faster in loading apps.
Also the Galaxy S6 edge has more advanced battery technology than the iPhone 6.
The Quad HD display of the Galaxy S6/S6 edge is really the main difference that actually crushes the iPhone 6.
The Galaxy S6's Super AMOLED display has more accurate colors than the iPhone 6's standard LED display. Furthermore, it's Quad HD which means the Galaxy S6 display is pushing 4X as many pixels as the iPhone 6. And it's more power efficient than the traditional LED display of the iPhone 6.
According to DisplayMate:
The new Galaxy S6 and Galaxy S6 Edge are Samsung’s latest flagship Smartphones. The Galaxy S series and Galaxy Note series are the way Samsung shows off its latest and greatest OLED displays and display technology. They have been alternately releasing one of these models every six months, so there are two OLED generations per year. Each new generation has provided significant enhancements and improvements, so they leapfrog each other in display performance, resulting in a new Best Performing Display with each new Smartphone generation.
The Galaxy S6 continues this trend, with display performance that matches or exceeds the Galaxy Note 4, which we measured to be the Best Performing Smartphone Display in 2014 along with the Best Absolute Color Accuracy.
The Most Accurate Colors
The Galaxy S6 Basic Screen Mode provides the most accurate on-screen image colors of any Smartphone or Tablet display that we have ever measured (effectively tied with the Galaxy Note 4). See the Galaxy S6 Color Accuracy Results, our Absolute Color Accuracy Comparison of the leading Smartphones, and also this regarding Bogus Color Accuracy Measurements. The measured Absolute Color Error for the Galaxy S6 Basic Screen Mode is just 1.6 JNCD, tied with the Galaxy Note 4 as the most color accurate display that we have ever measured for a Smartphone or Tablet, which is visually indistinguishable from perfect, and is very likely considerably better than your living room TV or any display that you own. Color Accuracy is especially important when viewing photos from family and friends (because you often know exactly what they actually should look like), for some TV shows, movies, and sporting events with image content and colors that you are familiar with, and also for viewing online merchandise, so you have a very good idea of exactly what colors you are buying and are less likely to return them. Select the Basic Screen Mode using Display Settings – it is not the default screen mode for the Galaxy S6.
What is particularly significant and impressive is that Samsung has been systematically improving OLED display performance twice a year with each Galaxy generation since 2010. With the Galaxy S6 there are many significant improvements over the Galaxy S5 that we tested a year ago, but also over the Galaxy Note 4 that we tested 6 months ago. The most impressive improvements for the Galaxy S6 are in the pixels per inch and resolution, the Absolute Color Accuracy, Peak Brightness, Contrast and screen readability in High Ambient Light, and in display Power Efficiency, which we discuss in turn below.
The jump from the Galaxy S6 to the Galaxy S7 probably wont be as much.
The jump from the iPhone 6 to the iPhone 6S probably wont be much either.
We'll probably need to wait until the iPhone 8 or Galaxy S9 to really see another huge technological leap from the Galaxy S6.
The 14nm of the Galaxy S6 is a huge technological leap from 20nm of the iPhone 6. Will we see 10nm in 2016? Maybe. But it's not 100% guaranteed and the technological jump from 14nm to 10nm isn't as big as from 20nm to 14nm.