I wouldn't waste my money on DT-40's because of the crappy aluminum coated mirrors (losing 10% power at 457 nm) with the high price of blue photons. Save your money for CT 6800 hp or better with much higher quality mirrors that don't lose power throught them, are designed better and last forever, wheras you're a guinea pig with Chinese galvos that are also very noisy and no track record on performance. You pay for what you get.
Cavest Emptor.
Mirrors can be replaced for right around $20.
http://www.laser-light-show.com/hardw2.htm
the 40's and 40pro's are quite different. You cant get 40's new anymore.
The Cambridge galvos are designed better, I'll give you that one. But they wont last forever, nothing ever does. I had a 6800 set die on me, the bearings flaked(post buy-out steel ball in raceway crown-lock type).
The RMA repair cost alone coupled with the average 8 week lead time could instead purchase a new set of dt40pro's. Furthermore: I can snag 2 sets of 40pro's for the cost of a single 6800 set.
I guess it comes down to what you really want/need. If I needed 60k for raster projection or some of those insanely complex shows bundled with pangolin, then Id go for a Cambridge 6215/amp set. If I wanted bragging rights or high resale value, I would once again go with Cambridge. But if I wanted a to do 30-40k at a price that wouldn't make me whimper if they flaked out during a show I'd get dt40pro's.
As far as track record goes, your absolutely right. I often think about how to increase the adoption rate of dt40pro's and get the subsequent track record therein contained. I still have a set sitting here for Bill Benner if things ever chill out over there long enough for him to play with a pair.