I like both as long as the Detroit was built before the advent of EGR. They're both great motors, and should have similar cost to rebuild. If you already have the CAT, I suggest rebuilding it instead of going and getting the DETROIT and having to get new wiring harnesses. It'll be worth your investment. If you spec the inframe kit right and do all the tricks (Pittsburgh Power ECM upgrade, OPS, MicroBlue, performance muffler, etc...) you'll get a fantastic truck that can pull your biggest loads AND get great fuel economy- even with a Pete 379 or KW W9xx.
Also, when it comes to truck engines, remember there is no replacement for displacement- a CAT C15 6NZ is a 15 liter, a Detroit Series 60 is a smaller engine (usually a 12.7 or 14.0 liter) so if you want to go with a Detroit for fuel economy, it's a good idea to spec a more aerodynamic truck along with the engine (the truck I would choose is an older (pre-2001) Volvo (like my WhiteGMC/Volvo W1A64T ES), but an older KW T800, KW T600, or Pete 377 (pre-2000) will be just as good).