When I first started knitting, I came on here and asked much the same question and got much the same answers. I decided, after some experimental knitting on different kind of needles, that I preferred metal, so I got a set of interchangeables of those -- also, I knew that I'd sooner or later want to try knitting lace, and people told me that I would probably want wood for that, so I got a set of interchangeables of those as well. (I'm traveling, you see, so I wanted to get everything I might need all at once.) I also have a few wood straights that I couldn't quite resist (Lantern Moon, get thee behind me!)
I'm really glad I got the interchangeable metals, which have been terrific and have used almost all of the sizes, and I haven't gotten quite to the point of doing the lace yet (just mainly lace-like and larger gauge lace), so haven't really used the wood ones much, but I'm glad to have them for the future.
I'd say that I've saved quite a bit of money, aggravation and time having gotten the metal interchangeables right away, since I'd done enough knitting on them to know I preferred metal. (If I'd been strapped for cash, I'd probably have gotten them separately as I could afford them rather than as a full kit.)
That said, some people knit almost nothing but socks or baby clothing or hats or what have you -- in that case, since many of those items tend towards certain sizes of yarn, I'd probably advise just getting those sizes at first if you know you're going to be that kind of knitter -- but that usually takes a while of knitting, to settle on one sort of knitting.