If you compare these two BTG slots, you'll find the game mechanics to be almost alike: Both have the Cascading Megaways with a max. of 117,649 ways, the paytable is identical, and on both you trigger the free spins by landing at least three scatter symbols (unlike Bonanza, where four are required).
WWTBAM has a slight edge over Extra Chilli, when it comes to average RTP in base game. In fact, the base game is the main reason to play this slot; it's highly recommended for wagering purposes.
The free spins however, are A LOT harder to trigger on WWTBAM than on Extra Chilli, and on average they pay less, at least when you consider non-gamble free spins. Personally I hate gambling for more spins, both on WWTBAM and Extra Chilli, and the gambling mechanism on WWTBAM is just too tilting for my taste. Those 50 spins seem practically impossible to reach - I only know of two players, who have actually managed to get there.
In fact, the gamble feature is the reason I'm only giving this slot a 6/10; it's just too scammy imho. Overall WWTBAM doesn't bring anything new or better to the table compared to Extra Chilli. It's supposed to give Max Megaways more often than Extra Chilli and Bonanza, but that's not my experience at all.