You’re going to hear mixed opinions all over concerning which block is “better.” AFAIK, there are more people making higher horsepower from the LSJ blocks using turbos.
In the end, I’ll be happier with the LDK/LNF simply due to the sheer amount of torque it delivers at low throttle with the VVT and the piece of mind of having a brand new engine.
By tuning being more difficult, I mean there aren’t as many tuning options on the Bosch E69 PCM. For example, the VATS cannot be disabled, which can be a pain in the ass later on. The fuel maps are extremely complex and do not offer near as much versatility as the Delphi PCM in the LSJ. You can read all about it on the HPTuners forum. Bosch isn’t terribly kind to the aftermarket.
With an LSJ turbo’ed, the entire process is much more plug & play with an SC donor and it simply leads to an easier build. Price wise, the turbo kits aren’t cheap, but its still a very reasonable bang for the buck.
On the other hand, to convert an N/A to run with an LNF/LDK block you will be nickel and dimed a comparable (if not greater) amount of money and it will be much more time consuming. To add another 100 hp to an LDK/LNF, you will definitely be spending more.
Another option is a pre 2007 SS 2.4 (if you can find one with a 5-speed). The pre-2007 internals are fully forged, make 175 hp and have VVT. There are turbo kits for those as well.