Mine reads both fine. But I have two friends whose machines will only read DVD-R. Also, at what speed did you burn? I burned a disc at 1X once and my XBOX wouldn't read it... I took the same ISO, same media, and then burned at 2X and my XBOX read it just fine. In my experience, my XBOX likes stuff burned at 2X the best. I've only tried DVD+RW once and it wouldn't read. Never tried DVD-RW.
Also, what brands are you using? I had some Memorex branded DVD-Rs and my XBOX and my two friends all read them just fine. Never a single coaster. But now I'm using TDK DVD+R and DVD-R (+R for me and -R for my friends) and the only coaster I've ever had was the disc I burned at 1X. I plan to buy Ritek DVD-Rs next since they are supposed to be highly compatible with the XBOX and can be found fairly cheap.
You may want to look here to see if your brand is listed:
dvdrhelp.com