Personally, I prefer straights for flat knitting. I find the problem with circulars are that the stitches often get caught on the junction between the needle and the cord and that you have to get exactly the right length of cord to suit your project if you're knitting in the round (I've tried Magic Loop - it doesn't work for me).
Having said that, a circular needle is good if you have a project with a lot of stitches in the row, as it holds them better than straights would. And they do work really well for knitting in the round if you get the right length.
There is a brand of needle (KnitPro?) where you can just buy one set of needles and then attach whichever the right length of cord would be for your project. So that could work.
I have knitpro interchangeable circulars and use them fir most projects now. The join is very good so the stitches don't tend to get caught up. The only niggle is that when I bought mine they didn't put the sizes in the needle tips so you have to store them carefully so you know which ones are which. I think they've changed that since though.
I have loads of straight needles but I only use the knitpro ones now. I'd only just got back into knitting, I didn't have any circular needles so decided to splash out on a complete set of decent needles rather than buy cheaper ones here and there. I'm less likely to lose needles from sets too. I always feel like I'm fighting straight needles, whereas circulars are much comfier and the weight of the knitting rests in your lap.