2001-2004 duramax trucks have injector issues. Gm replaced a bunch of them, but you will still most likely have issues after the factory update.
2004.5-2005 duramax trucks are nice. Some have the over heating issues.
2006-2007 duramax trucks are the nicest IMO. Allison 1000 6spd, tow mirrors, 14.40 stock 1/4 mile with 33s (I've tested this )
2007.5-2010 good trucks just lower fuel mileage
2011- don't know much, they have to run exhaust fluid.
CUMMINS TRUCKS
1996-1998 12v motors are great, good fuel mileage, mechanical pump, a lot of guys p-pump them. Biggest concern is the auto trans issues and front end issues. Also rust.
1998.5-2002. Injection pump issues. Rust on the doors, front fenders, wheel wells. Front ends are normally trashed. Decent trucks. No crew-cab offered. Dash's crack and fall the pieces.
2003-2007- great trucks IMO. We rarely have issues with these. Ball joints and track bars normally needs replaced at 100k. Dash's crack as well. Rust on rear quarters.
FORD TRUCKS
1995-1997. Solid trucks. Under powered compared to newer 7.3s.
1999-2003. CPS (this was recalled), glow plug issues, oil pans rust out ( to fix you have to pull the motor), bottoms of all doors rust, auto trans were the weakest point.
2003.5-2004- worst truck to buy IMO. Tons of issues, they did start the torqshift trannies which are amazing!
2005-2007- nice trucks. Some see egr issues which tends to cause other problems (including blown head gaskets). But one do a egr delete and arp head bolts and you will have a solid truck. A lot of injector issues known with these trucks are poor maintance. Add hot shot secret to you oil every once and a while and you will be fine. I would buy one of these over a 7.3
2008-2010- nice trucks. Twin turbo, tons of power. Most get 10-14mpg which is horrible. I had one that was on 37"s had a dpf delete, egr delete, 320hp programmer and it ran amazing and got 26mpg. The truck was crazy fast. Now some of these trucks had a radiator issue which caused other problems.