I have used both, and I really don't see much point in taking second wind over ER. I see both of their uses, but in current content I don't find a situation which would justify second wind for a protection warrior. 2% life/s is not very much, especially when you consider that tanks are not often below 35%, and when they are it should only last until a maximum of the bosses next swing timer (2s or so).
So a 500k HP tank will gain 20k life over 2 seconds with second wind, which is already lower than the instant 10% ER gives, with the addition of another 10% over 5 seconds. I can see the advantages of second wind though, in that it is a constant safety barrier with no cool down, and no reaction needed, but I'm struggling to see a 15-30k heal (roughly) honestly saving your life. I'm not often below 35%, and when I am I will already have a cooldown ready. I'm just asking myself does the situation where you are below 35% for a long period of time with no incoming heals arriving shortly actually come up often enough to warrant second wind?
I see people not using ER and I wonder why? Is it because it costs 60 rage or an enrage effect to be active (zerk rage)? What makes you choose SW over ER?