78 cu bottle...is that a 1.5?
1.5's are flipping huge, 1.1 is the perfect size, which is like 68cu or something
go for a 1.1 4500; good bottle.
otherwise as mikey said, if you din't mind about it's size get the bigger bottle...but if it's still 3000psi you won't notice such a large difference as if you went to 45000psi
hope that helps