The biggest difference is in the media - some players like only +R, some only -R, and some both. And some players like only certain brands on media - TDK might work, but Princo might not.
So, I wouldn't worry about the compatibilty aspect, except for 2 things:
1) You're right in wanting a dual burner, so if one type doesn't work, the other might.
2) Make sure you let any clients know that these aren't guaranteed. Some players won't play ANY recordable DVDs.