My personal preference is the 3.9. Having owned/worked on them all, I feel like the reliability and performance was far greater than either 4.3 or 4.0.
At least the 95 and 04 4.3's my dad had (the 91 being TBI) make 195hp and 260 lb-ft vs the 3.9's 175/225. So disagree wholeheartedly on performance. Every 3.9 I've run into was a total turd in the performance category. I remember a friend taking an 03 Dakota Crew Cab 4x4 3.9/auto to Milan Dragway and it ran a 19 second pass. The Blazer, the published times are 17.1ish for a 4x4 auto, not barn burner but a lot better. Weight should be fairly similar.
Reliability, I don't think either is too bad. More likely to lose a water pump on a 4.3 like any chevy small block but other than the lower intake gasket on the 95 (which was largely due to DexCool), they were all really good to us. The 04, I think in 12 years and 130k, my dad changed the water pump, belt, spark plugs, idler pulley and that was it, and he did quite a bit of towing. The 95 needed the CPI lines replaced at about 120k.
