I know a bit about the Isuzu's. However I was wondering if any one know what are the major differences ( Cummins V's Isuzu), many reliability.
Is that 'mainly reliability'?
If so there really isn't one (from what I've seen) if you are anywhere close to stock.
It seems to depend more (greatly so) on how they were operated and maintained. At least 4BD1T V/S 4BT. With the 4BD2 (series) there are reports of cracked cylinder heads. But then from what I hear, it's only a problem if you get it too hot which ties back in with how it was operated and maintained (dunno when it came in but the 4BD2-TC in my NPR has a feature that dummies out the altitude compensator to partially defuel the engine if it starts running hot, dunno if the later Cummins had an equivalent or not).
FWIW (and to be up front) I am looking into retrofitting the 4BD1T head (injectors, etc) onto the newer engine, because I want to tweak it up a bit. I have no way to proove that is a more durable combination, just rumor and speculation

(till somone publishes data on it).
Here:
http://4btswaps.com/forum/showthread.php?t=6754