there are mixed opinons.. but it all depends on what you are doing. i favor the iron block motors, they are only about 60-70lbs heavier, and bunches cheaper.
if you want something that has a bit of pep and you arent shooting for over 450rwhp get a 5.3 truck motor, you can get a cast iron block one for $600-1000 complete. aluminum blocked 5.3s are fairly common to because they were in all V8 trail blazer style suvs, but they will be more. if you are shooting for 450+ rwhp id go with a 6.0 LQ4 truck motor, you can find them for $800-2000 complete. with some $800 factory heads and a big cam through a stick will put down 550rwhp..
its all about what you want to do with your car.