I've not used the Harmony straights, as I pretty much do everything on circulars now. However, the Harmony interchangeables are great. They knit almost as fast the nickle-plated ones. I love them!
I would imagine the wood is the same on both straights and circs.
As to what size...what size do you normally use for your scarves? I find that it always depends on what yarn I use. Different patterns call for different yarns, which mean different needles....which is why I got the interchangeable set.