There are a million posts about both the regular Options and Harmony needles here. If you go to the search at the top under your name and type in "Harmony" or some variation of that you'll get a lot of hits and info on the needles.
As for what to buy..they are so inexpensive I suggest you get one of each to try and see how you like them. I bought a size 7 and 8 and 24 in cords (nickel plated) to test them and I loved them. It took me awhile, but I bought the set recently.
Those who have the harmony set seem to love them, too.