what is the benefit to buying Fortnite now when it is going to be free?
You get to play the horde stuff now, that's about it.
As for PUBG or Fortnite being better, I don't think there is a surefire comparison that's fair. With 18 hours into PUBG, I'd say Fortnite is a vastly simplified version overall. Movement, weapons and attachments, and overall level design is pretty barebones in Fortnite. The majority of it's depth comes from destructible buildings/interiors and the reliance on setting up fortifications. PUBG has a much bigger stealth factor too.
Yes, PUBG is more jank then Fortnite, but before the holiday break they cranked out 2 patches that did make improvements in about a weeks time, which is on par with how good Epic has been with their game. And if you weren't playing Fortnite BR right when it released, you probably aren't aware that it was actually in a pretty similar state minus the crashes. It had bad hitching on top of the very basic state it was released in.