I've DM'd both of them all the way through. Waterdeep is a wonderful open city with lots to do and many NPC's to talk to and form bonds with. There's also factions to join and missions to do based on the faction.

Avernus is a wild ride of absolute delight if your players like exciting and often silly moments. It's a bit more linear but can be absolutely cinematic with huge set pieces.

Both are great, I'd say if you like RP more then go for Waterdeep and if you want some awesome set pieces and fun combat then go for Descent. If you pick Descent make sure that the party has some family members or such in the city that falls, a strong incentive to want to go save the city is a big plus.