There's no DVDR. The original format is DVD-R, written like CD-R. DVD+R is an update to the original DVD-R format. The main change is slightly better initial access times, and a better error correction. Since DVD-R already existed, they just changed the minus for the plus sign. DVD+R is more recent, so old DVD drives (both for TV's and computers) will not recognize the +R discs, nor will be able to write to them. Finally, DVD+R is a propietary format which is not exactly part of the original specifications made by the DVD consortium, although the companies behind it all are part of such consortium.
Choosing one is easy. DVD+R gives you slightly greater performance, but don't think that DVD-R will corrupt your data or something. The difference probably is only noticeable in borderline cases. DVD-R is now slightly cheaper (at least around here), and has better compatibility. Just choose your priorities.
As for your Memorex DVD's, I'd swap them for other DVD's with someone else. Chances are that they're just incompatible with your toaster. I've had the bad luck to see many disc-burner incompatibilities, even when using good brands. It's just a matter of luck, chances are that the discs are in perfect shape, but just not usable with your exact model of burner. I was once talking with a MPO representative some years ago (MPO is a big digital reproduction company from here) and he told me they were developing their own CD's and they worked with all burners in market except *one* and they didn't know why. It's just like that, too many combinations, some will always fail.