Big block would be the easiest. Small block would be cheaper.

Remember big block will require new trans core (different bell housing)
Distributor
Pullies
Brackets
Water pump
Timing cover
Etc...

All of that stuff will work off your current engine without "tearing" it up too bad.

Big block Eddy head will KILL most of the out of the box small block heads though. So a 500" big block will make it to 10.50 easier (less radical combination), then a 400" small block will.