The Vesper has a single sprag clutch, the classic hub has two. If you ride hard, weigh more than 170, ride loaded, etc, just get the heavier one. We've had no durability issues with either hub, but then, we spec the hubs to the riders. They are the best engineered hubs on the market. Silent, actual instant engagement, but more importantly the drive mechanism is behind a set of sealed bearings and a labyrinth seal, not behind some little rubber o ring. DT hubs have fine engagement, but they have a critical flaw. The smallest, fastest wearing bearing in the hub is in the freehub body, and it's not supposed to be user serviceable. If you do get it out, (which is possible) you will spend loads of time swearing. Yes, DT covers it under warranty, but they do so. just by sending you a whole new freehub body. That's kinda nuts. I bet that most DT hub riders stick their finger in their hub, feel the outer bearings, think: that feels fine, and go on riding with a crummy, crushed, little freehub bearing. I've never taken one apart that was more than 2 years old and had actually been ridden, that didn't have a bad freehub bearing.
The best hub out there is the Onyx IMO, followed by the under rated Bitex touring hub, which is as good as any hub is going to get for under 150 bucks.
-James / Analog Cycles / Wheel Builder for the past 20+ years.