5.7's are cheaper than 6.4.
Stock HP though as you know, is well shy of your goal, so youll be into mods on the little wonder work horse. bolt ons alone (headers, intake, etc) will not get you there. Youll need long tubes, cam, engine/head work, tune - if running oem PCMs, etc.
6.4 is already 485hp, minor bolt ons will put you nicely past 500. It takes more to make the same % of power from a 5.7 than a 6.4 thus the trend towards going that route out the gate. With hellcat conversions being all the rage, 375hp seems sorta pathetic now days.
If your building up a 5.7 or slightly hopping a 6.4, (again, to meet your stated goal) all the rest (cooling, elect power needs, driveline, fab work, etc all would all similarly fall in line nearly the same. ie: same work & money eitherway you go...so little to be gained there on the back end, by going with the 5.7 once the initial engine is purchased. imho.
6.4's though do have their weaknesses (thus thier nickname said in hush tones) but also have a solid rep for building great power & tq. within thier design limit.
so I guess it all depends on whats important, how much "turn key" do you want (no such thing here, but you get the idea I hope) and how much $$ is in your wallet, you can get there with either engine. best of luck.
