[Haskell-beginners] Question on Lists

I have been reading Chris Okasaki's PhD thesis on Purely Functional Data Structures (www.cs.cmu.edu/~rwh/theses/*okasaki*.pdf) and it discusses his idea of lazy lists (He uses Standard ML in the paper), and it raised some questions for me. To cut to the chase, my question is this: Say I have list x of length n, and I have a single piece of data y. Does x ++ [y] take more cycles, or the same as if I'd said y:[x] (which would get it on the wrong side?)