They're both games for different markets, different genres.
Dauntless is a co-op monster hunting game, similar to monster hunter. Has a bit of an issue with microtransactions from what I've heard. I don't know terribly much about it, other then it kinda just looks generic-y free to play.
Warframe on the other hand, I think I have a good amount of time in (about 500 hours, my most played game on steam) to be able to give a good overall opinion on. The microtransactions are there 100% as a "donate if you can and if you want to help the devs". The single premium currency in the game can be gotten through trading with other players (no scams, the currency can be traded via the trading menu along with the items), and everything can be gotten in-game through playing (except for a handful of 100% cosmetic-only items which are generally either player-created skins or things like that).
Warframe as well has a few players here, on both PC and switch, so that's perfect considering the major flaw for warframe is that the learning curve is incredibly steep, and that's where most people drop off if they don't have seasoned players to help them through. It's worth that push though imo, since you'll join into a long-lasting game community, with devs that have constantly poured love and time into the game year after year, with the updates only getting bigger and bigger.
Warframe does have grinding to it, but due to all the constant quests (pretty sure nightwave is on switch now), loads of different types of missions, and also the 2 huge open-world areas (more gonna be added
), there's enough variety to not make the gameplay go stale. Different weapons, different mods, different stances for melee and even tons of different abilities for the different warframes to play around with and find your style.
Warframe's story is also bloody awesome, it's really deep, the singleplayer cinematic story missions are stunning and beautiful and damn nice in terms of game design. There's no end in sight for warframe, it's just gonna be updated for as long as Digital Extremes can go. Space mum will protect us tenno forever <3