PUBG is more "realistic" battle royal with a bigger map, gun attachments, armor (Fortnite does have shield potions though) more weapons, first person view, vehicles, bullet accuracy and gun handling. It also has two maps vs one (at least on PC, not XBox yet) The downsides are It's $30, doesn't hold stable 30fps, lots of bugs and looks ugly.
Fortnite is a free battle royal game with a smaller map, can be locked 30fps on the X (or unlocked where it roams between 45-60fps) and is much more stable in terms of perfromance and looks appealing despite it's cartoony nature. The downsides are halo reach bloom like bullet detections, less weapons and no vehicles compared to PUBG, no first person view and of course no gun attachments. I don't know if they are adding gun attachments in the long run or vehicles.
They both have their advantages. I'd say if PUBG was on the PC full release schedule with the extra map, bug fixes and a locked 30fps performance it would be the better pick. Sadly PUBG is in a buggy state, though they are patching it once a week so over time it should be a much, much better product.
If you never tried a battle royal game before Fortnite is free and free is always good. Don't know when they are adding the free trial to PUBG. I'm assuming when sales start to slow down on X1, if they ever do. Heard It's already over 4+ million on the XBox.