To my recollection, EBS has only suffered one such major failure, some time in 2011 (I was working for Engine Yard at the time). It hasn't factored strongly into my decision making since, other than that since AWS weathered it, they're likely now well prepared against it.
AWS is a known quantity to me, Joyent is unfortunately not. I have a lot of time for Bryan Cantrill though, so I'm sure they're doing good stuff - I just don't have the extra cognitive cycles to handle a whole new base layer on top of all the scheduler / container stuff right now.