Both are just lists of points to be visited in the order they're listed - so I agree it's a bit confusing to have the two choices.
Each have pros and cons, but a Track is simplest in concept because it is essentially exactly like having a highlighted paper map on your handlebars. It's better than the paper map because it scrolls as you travel, and is zoomable to taste, and is wind/waterproof - OTOH it's not so good because it's dim, and very small.
If you choose this option, the trick is to colour the Track(s), in the GPS menus, to make it much more visible on screen, more like a highlighter pen, and to observe the 500-point limit by splitting and/or downsampling.
A Route on the other hand is more analagous to a turn-by-turn route sheet. If (like me*) you are used to riding with a route sheet and never refer to a map, then I would suggest that learning about Routes will be more rewarding in the long run.
* for example - if someone were to ask me the way to my house, my instinct would be to write "leave the A6 here, then right here and left here" - rather than to draw a sketch map. That seems to make me a Route person not a Track person.