I recommend the aluminized stuff, but I'm cheap and don't see the benefit of the stainless for the diesels. If you had a gaser, I'd say otherwise.
Now. . If you want to show your truck and like to spit pollish everything on it, then the stainless is the way to go.
Aluminized pipes will last a long time behind a diesel engine. Probably longer than you will keep the truck. If you know for sure that you'll keep the truck forever and will never want to make any changes to your exhaust system again, then go for the stainless.
I've seen 4" aluminized systems on Michigan school buses last over ten years, except for the mufflers, which tend to rust out under the hanger straps that wrap around it.
Doc