Most commercial movies are dual layer. That is the whole reason for compressing them, is to get them to fit on a single layer disk. I have found that most will compress down enough to fit, and for those that don’t, you can usually just remove a few things (I often just remove the foreign language audio tracks), and then they will fit. A very rare few cannot be compressed enough unless you take a lot of the extras out (often ones with non video extras like games on kids movies etc). reasons to use dual layer would be for instance, a movie that absolutely will not fit, or maybe one that you want all the extras, or perhaps its your favorite movie and you want the best possible quality (compression reduces quality a little bit).
As far as verbatims, they are the only good dual layer disk that you will easily be able to find in the US. Single layer verbatims are good too, but there are a few other good single layer disks also.
On the bitsetting, you have to have a drive that is capable of it (many are), and yes you can generally set it in software. Some drives default to bitsetting, while others you will have to set it. After burning a disk, you can check it out with cdspeed or dvd identifier to verify that it did in fact get bitset (you drive may already be doing it by default).