Well, here we go. I happen to have the same exact flavors as you do. Just my experiances, my 89 is a 2.6, top end blown when I got it for a $150.00. Slick body, good interior, frame, floor bords, etc. I did'nt know a thing about Isuzus at that time. Notice I call it the money pit. Parts are high. That said, I picked up a 90 2.8 rust bucket for free that runs like a top so I'm going to drive it though the winter so I can finish investing my grand kids college funds on the 2.6.
The 2.6 has a tough bottom end, (reminds me of a john deere), IMHO, more fun to drive. Down side is it's a zero contact engine which means if the timeing belt goes, you're gonna have some bent valves at the minimum. Early head castings are prone to crack, etc.
The 2.8 is the pride of G.M. Leaks oil no matter how many gaskets you put in it. Pulls better on the highway, but the two things I really like about it is if you pop a timeing belt, you're just out the belt and upgrade options are there.
What I should probably do is get rid of them both and find a slick 2.8 first gen. But alass, I'm stupid.