Completely different cars for different purposes. If you want an SUV don't be tricked into thinking the Carnival is one. It's not. A more extreme is comparing a Jeep vs a S class Mercedes but the Mercedes has the same level of comfort for all passengers and the Jeep has an extra row for more people (though they may not be as comfortable). If 99.99% of your driving is on a road in fair/moderate weather conditions, the Carnival is a great choice. Even if it snows, snow tires on the Carnival will significantly out perform AS on a Telluride. If you want something that you can periodically go off road or in the rain/inclement weather have confidence in pulling out, up hill, in traffic, without having to change the tyres, then the Telluride is the way to go. If you want to pay $7-$15K over MSRP, Telluride. $0-$4K over MSRP, Carnival. Blind spot assist on the Carnival (especially the SXP) is fantastic. I think the Telluride is as well but don't have first hand experience. Last, the Carnival swallows a ton of luggage without compromising the enjoyment of passengers. It's simply amazing how much you can stuff in this thing. If that's not a factor then the Telluride may be better.
Good luck and keep us updated with your decision process.