Fournette's NFL production is obviously a complete unknown at this point but taking him before Gurley is no stretch. In my 1/2 ppr, Gurley was the 19th best RB last year so nothing special.
While Gurley showed flashes of greatness his rookie season (including several big runs), you also can't ignore the fact that he averaged 3.2 ypc over the course of season last year. In fact, since the midpoint of his rookie season, he leads the NFL in carries but ranks last in ypc (minimum 200 carries). As mentioned, he also cracked 100yds only once since then and had zero runs over 25yds last year which accounted for a decent portion of his success his rookie season. While he looked great at times early on, you can't ignore the lack of production spanning multiple seasons. Pro football focus has an interesting article on this...
I also wouldn't bank of the Rams offense being much improved. Fisher gone helps and so does Whitworth, but the QB situation is horrible (Goff didn't even show glimpses) and the Rams did nothing to add playmakers to pair with Gurley. I wouldn't be counting on the rookie WRs and TE they drafted to contribute much this season. They did add Lance Dunbar, though, who should eat into some of Gurley's receptions next season, similar to Cunningham's role the past few seasons. I hope I'm dead wrong as I own Gurley in dynasty, but we saw Gurley's floor last year (I again hope), and it was scary.
Fournette, on the other hand, is on a better team with more weapons and with a coach who has committed to running the football. They also didn't draft him that high to sit or split carries so while I certainly see others getting the rock to spell Fournette, he projects to be the guy. His role out of the backfield as a receiver remains to be seen. Even with the uncertainty around his role as a receiver, he will get enough carries to challenge Gurley in fantasy production and wouldn't fault anyone for taking him over Gurley give his recent track record of mediocrity.