The best dual layer media I have found are Verbatim DVD+R DL (made in Singapore). I have used Fuji, Sony, TDK and Memorex DLs and found them all to be, without question, inferior to the Verbatims. The Memorex DLs were the most frustrating media, producing coaster after coaster.
I used a BenQ DW 1640 and a Plextor PX-640A (rebadge of the BenQ) along with ImgBurn when using these various DLs, so that the media seemed to be the limiting factor. With the Verbatim DVD+R DLs I almost always had success in burning troublesome dual layer media.
At present I am using an Optiarc AD-7241S, ImgBurn and Verbatim DVD+R DLs with superb results–and not one coaster (he said with fingers crossed).
Although the Verbatim DVD+R DLs are somewhat expensive, they frequently go on sale. Check CD Freaks “Bargain Basement” along with Newegg and Amazon.
When you factor in the cost of coasters from inferior media, the price of the Verbatims is–not cheap–but quite reasonable.
Good luck with your purchases–but with Verbatim DVD+R DLs you won’t need luck. (No, I do not work for Verbatim!)