The most important is compatibility:
I have tried at least 10 times to burn DL discs, using 4 different brands. Outcome:
the DL discs are only readable using the burner that wrote it. That aplies for all brands. All other pc drives refuse to read them. They play fine in my Pioneer DVD player, but on two other deck players I tried, only the first layer was playable, afterwards the player froze.
In two cases (different brands), the layers got seperated when I accidentaly hit them (that will hapen in post office, trust me) and then they were unplayable.
If you try to burn your own DL dvd, you don't have to worry about layer break, Nero, alcohol, DVDdecrypter and Elby clone DVD put it automatically if you choose not to specify it manually.
But even if (for a reason I can't understand) the layer break is placed in wrong position, the only disadvantage will be a 1 second still image at the half of the movie, ie no big deal.
there is no technical knowledge required to burn a DL disc. You just copy-paste the files contained in the two discs in one folder, and then burn them contained in a folder named VIDEO_TS.
That's all.
So, my point is: Leave the guy alone here, and settle with the two SL discs to avoid more trouble afterwards (I'd hate to have him send me a DL disc -which after all takes ages to be written in comparisson to 2 SL discs- and then ask him to re-send me the discs cause the one I got was not compatible).