I've only ever fitted double extension when I needed regular access to things at the very back of a drawer. The only example of this was CD's that needed to be lifted out vertically as the drawer was packed tight from front to back.
Double extension is the way to go.
Have just built a stand for the coffee machine with a drawer for all the "bits"
The question for me was whether to go side or bottom mount. Decided on bottom mount because it was easier to get the alignments right.
Double extension drawer slides will have more storage space than ordinary drawer slides. If you need to install them, double extensions are a better choice.