I see a lot of stuff being tossed around here that seems to me more like what people have heard from other people about stuff. I will talk on what I have specifically come accross with between these 2 motors.
I cant speak on the 3126. I can speak on the C7, witch is an evolution of that motor, just as the Maxxforce DT is internationals Dt466 for now a days.
now, let me talk 466s. I have seen more then a few trucks that are over 300k, and have not been inframed yet, so they will last past 150k. The 12valve non EGR motors feel stronger then the newer lot of trucks rated at the same power, no idea why, just most of those DT466E's and the DT466s I have driven in box trucks and school buses have been strong VS the Maxxforce DT generation trucks and the EGR DT466s.
When the DT's went to a 24valve set up, they do occasional have problems with failed camshafts, the lifter holder will break, then the lifter will spin around in the hole, eat the camshaft a bit, and then cause a misfire. It is a big job, and takes a lot of parts. A cam job is just an inframe minus doing the liners and pistons rings and such, so weigh in on that. The dt's will occasionally and rarely drop a valve, and then require an inframe or a motor job because metal will pump through all the cylinders and damage the liners.
The C7 I have only dealt with on MRAP applications, and for the most part, ran like champs no real failures. One thing of note, was that the Maxxpro MRAPs also run DT motors, and also I did not hear of any of those motor having failures, so maybe my C7 experience cant be taken into account. I do know, they run strong.
I would recommend trying to get a truck, witch ever motor you choose, that runs a 3000 series allison. They really wake the trucks up performance wise vs the 2000s. The transmission means business.
These 2 motors mechanically are to similar to say 1 is hands down better then another, just a preference thing, like picking a trucks color. They both have there strengths and weakness.