I have used one of the Sony dual format drives for a while, as have two of my friends. We've been happy with them. Sony has done a good job of releasing firmware updates to give speed improvements. Even at $300 it was a good deal.
As with everything else, it depends on what you want and how long you expect it to last. If the aim is to create DVD videos, the -R format is supposed to have more compatibility with "normal" DVD players (i.e. connected to your tele rather than your computer). Having said that, most modern standalone DVD players should be happy with both - mine certainly is and it was bought over 3 years ago.
Certainly the single format ones are cheaper than the dual format drives. +R seems to be more popular than -R and media is often more available and can work at higher speeds.
If it were me, I'd spend more and go for a dual format drive. If you just want something and expect to replace it in a 12-18 months (when they'll be $50) then get whatever is cheapest.