I've heard great things about both the EZ and the Diesel, so the EZ is definately not a bad boat. They are completely different boats though. The Diesel is one of the few modern big river runners/creeckers/anything-big I've had the pleasure to try, and it hasn't been disappointing. It's a great boat, it's as stable as a house and rolls like, well, something that rolls (although I've heard complaints from someone else that she found it quite hard to roll since she learned rolling in a smaller boat and the technique was a bit different). So it's definately not a bad choise if you're looking for just that edge in stability and confidence you need to take the next step.
However, nearly drowning does not sound like simply taking the next step. A diesel, or any other boat, will not prevent any nearly drowning. Of course I haven't heard the full story, and I wasn't there to see it happen, but aside from your boat you might also want to take a second look at the way you run rivers. I'm not saying you should go do something easier, since that's probably not what you should do, but perhaps you could spend a bit more time scouting rapids, looking at them, perhaps getting out of the boat a bit more often to do so, look for the lines and see how other people do it. If there are hazards in the course that could nearly drown you, you can look for a way to avoid those. Perhaps you could draw the attention of your groupmembers to that spot, just ask them what the plan would be if someone would get into trouble over there. And if you really don't thrust a rapid, just carry your boat arround. This will help you just as much on the road to not nearly drowning as getting a Diesel, even with it being a great boat.