The differences between one 4.8 to another 4.8, or 5.3 to 5.3 is minimal. Biggest difference is generation and block material. Going gen 4 you get a better rod, arguably stronger with floating wrist pins. Gen 4 engines also typically run a different crank, cam, and knock sensor configurations then a gen 3, but is not usually a big deal just something to consider when choosing a correct ECM and harness. When it comes to 6.0 engines the big difference beyond generation id if it has dished pistons or not, the most popular engine the LQ4 has dished pistons, LQ9 has flat tops, everything else is the same. Get into newer stuff and there's a ton of different RPO codes and variations, but things are easily adapted no matter which one you start with.
In my opinion it's a toss up between the three truck engines, the 4.8 will easily rev with a cam change and it's usually cheaper. The 5.3 is plentiful, cheap, and very versatile. The 6.0 typically costs a few bucks more, but you run bigger heads and get more out of it. An aluminum block is great, I won't talk it down, but eveyone seems to think they are worth a lot more. The weight difference is about 88 pounds between the iron and aluminum blocks, I honestly don't see that being a big difference maker in performance for most people. Otherwise the two blocks are basically identical in design. If I had a lightweight car I think I high spinning 4.8 would be fun. If I wanted more power a boosted 5.3 could make over 500 wheel and be daily driven without any ill effects.
You'll figure out what you want...maybe....or I suppose this is Grant's flavor of the week this time.