I have all the ETS content, too, but I haven't tried it yet. One question I have is how long are your deliveries compared to ATS? Are there deliveries that span the entire map, and how big is the map in real gaming terms compared to ATS? Personally, I'm having more fun with the longer journeys.
Another question I have is, and this is relevant to both games, is do the truck brands behave differently in terms of steering and tipping over at high speeds around corners, are all the trucks the same outside of looks? So far, besides the really low end day cab with a teeny motor, the only high end truck I've driven is the Lonestar, so I don't have anything to compare to.
I play much more ATS than ETS, so I still ain't sufficiently qualifiied to answer your questions properly (maybe some ETS vet will pitch in), but I can still tell you that ETS, being around for longer, has had a lot more map expansions and the current map (with all DLCs) is gigantic (with more n the way, next is the Iberia peninsula). Besides the horsepower, I confess I never noticed much (if any at all) difference between the trucks in terms of handling.
I think the biggest difference between playing ATS and ETS, is mostly the scenery. ATS has the typical american long haul vast expanses, sometimes dramatic landscapes, a slighly bigger sense of isolation, and much more space between urban regions. ETS on the other hand, has more diverse landscapes, it is also more "compact", with many villages between larger towns, overall it's more "green" with many forested regions (but this can change a lot between parts of europe), it has a lot more visible arquitecture differences (because it spans a lot of different and very old countries). It also has ferry rides, which are very cool.
ATS features mighty Mount St. Helens (you can actually drive into one of it's main viewpoints parking lot), but ETS features mythical Mount Vesuvius and Mount Etna.
Both are bloody excellent, while I do tend to play ATS more because of the change of scenery and the cooler trucks (IMO), I couldn't decide which is best.