US: Heavy-duty: Emissions | Transport Policy
The US has phased in progressively more stringent regulations for diesel engines since the 70s. The big one starting in 2004 was a significant reduction in the standard for NOx
emissions (the stuff that causes smog), which led to the introduction of EGR (exhaust gas recirculation) which was the bane of many bus mechanics' existences (and of skooliers who buy buses from 2004 or later).
One article I read some time ago said companies generally phased in the changes on their assembly lines throughout the calendar year 2003, with some companies starting in late 2002 and some in early 2004. There's a lag of some time between when an engine is built and when it ends up in a bus, so with a 2003 or 2004 you probably can't tell if it has the EGR just from the year (although a 2003 probably doesn't and a 2004 probably does).
I thought the DT466 changed its name to the MaxxForce when the emissions controls were added (I don't know if that's the case) so perhaps the 2004 is an emissions-free engine, in which case I guess the 3060 tranny gives it the edge. I'd still go with #1 since it's fine and cheaper.