I own both the Utopia and the HD800S, and I faced the same decision. Some said the Utopia is better, others the HD800S. But whatever you do, I'd trust Focal to take care of you in case you have a problem, at least they did in my (and many other) cases.
My take: The HD800S does one thing exceptionally well, and that is classical / accoustic music. It is tuned that way and that is where it will shine. And you will need a good amp, the mojo (which I also owned) will drive it but you stay a long way under the headphone's potential (as would the Utopia, btw). So if you are 100% classical music listener - go for the hd800S and spent the rest of the funds on a tube amp.Or the ifi micro black label worked great for me as well with the HD800S. But if you are looking for an allrounder, the Utopia is definitely the more satisfying experience. And yes, there are driver failures, but the Utopias are so exciting that it is worth taking the "risk". The mojo will be fine for the beginning, but it will only shine more once you have a better source component.