you will have to use hacks to get an external drive to work with apple's iLife, if the drive will work at all.
go with the internal. why have an external on a tiny notebook? it would be practically bigger than the laptop. you going to cart the drive around?
do you REALLY need a dual sided disc? they cost more than twice the cost of a single sided disc. where is the benefit? you add 2mm by burning to two discs instead of one, so space is not a factor. you can lose one, and still have the other if you burn 2 dvds, you are SOL with a dual sided disc. labeling is no good on a double sided disc...