Re: race engine
Depending on what you're looking to do, the 4.3L versus the 3.8 is an interesting debate. Here's my objective take on it:
Heads-Vortecs availible for the 4.3L, modern chambers good flow characteristics and larger valves when compared to the stocker Buick heads. Otherwise you're running ported stockers or GN1Rs. Brodix -8 and -10 heads are availible for the 4.3L if you're lookin for crazy power. Price/$ winner:4.3L
Displacement-Cubes do help. Winner:4.3L
Intakes-Getting a good port fuel injection intake for a 4.3L is a PITA, short of getting a rare/expensive Sy/Ty intake or converting something. Winner:3.8L
Rods-4.3L rods are nowhere near as nicely engineered as buick rods. If you look at them, you can tell that the Buicks are quality. Expect to buy some H-beams for making a good bit of power with a 4.3L reliably, and the SBC rods do not work. Winner:3.8L
Pistons:4.3Ls use SBC pistons and rings, perhaps the easiest item to get and a shelf item none the less. The Buick stuff is a shelf item, but at the same time nowhere near the selection of the Chevies. Winner:4.3L
Engine Management:the 4.3L uses a Speed Density system with probably one of the best documented factory ECUs out there. The Buicks have a huge aftermarket of chips, and a 1227749 box from the Syclones/Typhhons will retrofit into the 3.8L Buicks. Winner

ush.
Cam:A 4.3L is a roller motor stock, the Buick 3.8L needs expensive retrofit parts and are known for having some issues with cams and eating the #3 exhaust lobe. Winner:4.3L.
In the end, I think that both motors would be fairly similar cost if you're doing a built short block, but if you have cheap machine shop time to do a modified or custom intake the 4.3L would come out on top.