6.4 block is beefier than the 5.7. The 5.7 has a cast crank; the 6.4 crank is forged. 6.4 is a much better platform to start with. Add some "drop in" forged rods / pistons and you have a real nice short block.
My opinion is if you're going to swap pistons and probably rods then you're not far behind a drop in stroker kit --- could get a forged rotating assembly and drop it into a 5.7L block and have a 6.4L... You also know the exact components within the engine. Gen III heads are out there and super cheap... I find 6.1L heads for like $400 a pair rebuilt. Can get random cores for like $150 a pair. I've been thinking about doing something similar to get a forged short block to swap under my 6.1L top end for boost later on.
Anyway - you could go with a factory 6.4L but by most accounts they are made of glass when it comes to power adders... the weak points are the mentioned rods and pistons... why drop more coin for a 6.4L when you can build a better short block for around the same money
