Both have their good points and bad points. DPNs are great for small things knitted in the round. For those who are intimidated by using more than two needles the alternative is knitting small circumferences on two circulars instead or using Magic Loop, both of which are fine alternatives.
Circulars are also best for larger circumferences, such as sweaters and bags. DPNs are really good for socks, or two circs, whichever is more comfortable for you.
I did have two socks on two Boye #2 circs (seaweed toe-up) and I transferred each sock to a set of DPNs. The Boye does not have a smooth join from cable to needle and I was wasting a lot of time trying to get the stitches from cable to needle to be worked. Until I can get some circs with smoother joins from cable to needle I'm going to stick with DPNs for socks. I've heard a lot of good stuff about Addi Turbos so I might look into those when I have some more funds since they're on the pricey side, but seem to be worth it since I have heard very little negative stuff about them.
I do know for straight knitting I'm starting to prefer circs over straights-they're easier on my hands and wrists. I also like the bamboo DPNs since they aren't cold so I might also look into bamboo circs as well.
Anyway, everything you see written here is the opinion of me and me only, YMMV.