My 2011 was great for two years. My 2015 is less than a year old and its been flawless as well. But newer is always better in my opinion.
In 2015, they got a little more power. Mine is now 440/850 I believe versus the prior 400/800.
I've had them all: 7.3, 6.0, 6.4, and the 6.7. Going from a 7.3 to a 6.7, you will almost wet your britches the first time you pull the camper. Your trusty old 7.3 will become a sled in your memory rather quickly.
2015 F350 6.7L Lariat CC LB
2016 Vengeance Touring Edition 40D12
Former Forest River Campers:
2004 Puma 30FQSS
2010 Sabre 31QBDS
2014 Work & Play 30WRS