I've had all three. S6 edge, S7 and now the S7 edge. In addition I've had the S6 edge plus as well.
Out of all these phones, the S7 and the S6 edge plus are the most comfortable to hold. S6 edge plus because of the sharp corners. While it maybe uncomfortable to hold for some, it provided extra grip. S7 is an extremely ergonomic phone. It sits naturally in the hand. But the rounded corners still didn't allow for much grip. I've used both of these phones without covers without any issue.
S6 edge with its sides have been too sharp for me. Although using it without a case hasn't been much of an issue either.
The S7 edge is a whole other story. I don't know what it is about this phone, but it's too slippery at the sides and I don't feel secure using this phone without a case at all. And probably won't ever do it. The side rails on the S6 edge and the S6 edge plus stuck out a little from the phone with a slight chamfer. However, with the S7 there's a continuity with the front and back curved edge with the metal rail. While this looks good, it doesn't feel very good. Especially with the accidental touches and grip while navigating the phone.
So in terms of ergonomics I'd rate them as follows
1. S7
2. S6 edge plus
3. S6 edge
4. S7 edge
However if it was me, I'd still go for the S7 edge. Because with a case, all this discussion goes to waste. I have a Spigen thin fit on my phone. It provides plenty of grip and a fair amount of sides to avoid accidental touches.
So, my advice would be to get the S7 edge in coral blue or wait for the glossy black. And then put a case on it. It will be worth it.