Six of Crows is definitely cool! I’d say go with Rivers though because if you do like it, there’s a giant pile of sequels, whereas with Six there’s only one.
As an aside, Netflix is adapting Six of Crows and the author’s Grisha trilogy together as one TV series because they’re set in the same universe (and eventually crossover a little). It’s kind of an interesting approach. The Grisha books are earlier chronologically (and a very different style - a sort of gothic romance rather than an awesome heist), so instead of like doing one after the other, they’re doing the “early years” of the Six of Crows characters (which in the books are only slowly revealed as needed in flashback - and never excessively, unlike most flashbacks in books!), which are admittedly pretty wild in some cases alongside the Grisha stuff (even though some of them are in “fantasy Russia” and some in “fantasy London/Amsterdam”).
I can’t think of any time where anyone has tried to do this before with same-world-set stories, like recombine them and run parallel tracks. I’m unsure how well it will work, because the Grisha stuff is almost for an entirely different audience to the Six of Crows stuff, and nailing both tones at once will be hard. It’s coming out pretty soon actually - April, and called Shadow and Bone, after the first book in the Grisha series. I give it 50/50 as to whether they fuck it up or not. Actually given the showrunner has picture of Kaz Brekker (main character of Six of Crows) as his Twitter profile pic, maybe it’s slightly more likely to succeed than that. Also the casting is 100% spot-on.
(And the same author’s recent modern-day-magic book, The Ninth House is also being made into a show, but by Amazon - jeez!)
Rivers of London is also total adaptation-bait, and it’s been optioned and people have nearly made a series a number of times, but it has yet to happen. It’s bizarre because it would be such an obvious shoe-in for a BBC show. That said Nick Frost/Simon Pegg have it now, and were talking in 2019 about how they wanted to do it after Truth Seekers, and that only just came out, so perhaps it’s still on the way.