Technically speaking, Decal is the overall term, (According to Wikipedia) Which includes Transfers, AND Stickers. From my days marching around and playing with Fighter planes for real, Decals is also the term used for any marking that is pre-prepared and applied to the aircraft (as opposed to painted on markings from a stencil).
So I guess, Decals is a broader term, and transfers is a more accurate term. But Decals sounds more grown up.
I watched a Video on Youtube the other day, where the narrator constantly referred to Decals as "Dickles". The pronunciation was so distracting, I actually cannot remember what the video was actually about. Possibly an unboxing, but not sure.
Personally I prefer Dee-cals, although Deckals is probably valid too.
Transfers is just how I end up paying my Credit card bill after a big month on building the Stash!