One old trick is to pull a certain freeze plug and look at the casting on the outside of the cylinder for a "428" cast in.
Everything else that folks have already said is right on.
Rare? Desirable? Collectible? Each has value.
Between a 428 and the 427, IMHO, I guess two high value base lines could be:
1) An original unmolested, standard bore 428CJ meeting all expected specs for a Mustang or Shelby like a GT500. That would include matching numbers block, crank, heads, manifold, carbs etc.
2) A 427 s/o in same condition
Adjust down accordingly from there.
If I had those two in front of me, I would give the 427 my personal choice. Unless, of course, I was restoring a car or THE car that the 428 came from.
Too many variables in play for someone to be able to provide a question to give a simple answer!
