If it helps any, I think Rhythm Thief does seem like the better game just from its demo. I haven't bought the game though, but I plan to eventually.
Do it! For me, the game that could me hooked was on the demo when you sneak through the Louvre hiding behind statues. I loved the music and design. The other games were interesting enough for me to take the plunge. In the end, there are 50 rhythm games but most of the concepts are done twice and in some cases 3 times. So, there's only around 20 different types and you might like some more than others but I found it a blast.
I want a sequel and this is definitely something I think could be made into an awesome franchise. Personally, I think they should have Phantom R visit different locations around the world for each sequel. So, a game set in London, a game set Moscow, a game in Rio De Janiro. And then you go and explore the city and all it's famous landmarks in the story as well as having a chance to infuse a bit of the local music into the game and rhythm games.
The only issue with that is the first game establishes most of the characters as being Paris citizens and I think it would be hard to move them to different locals. At least Vergier and Charlie. Marie could be moveable in that she could be touring to perform if you really need her but Phantom R could be a James Bond type and meet a new girl every adventure as well. Otherwise, I think it could be sort of stale to always be set in Paris.
Rhythm Thief is just too unforgiving. You either have it or you don't. It rarely allows for that middle ground you need for a Rhythm game.
Perhaps. I do think there is a shift in the degree of difficulty around the Chapter 7 point. Although even that can still vary. In the final fight with Napoleon, I A ranked that game first try but my first fight with him I only got a D and it took me a few tries to get to A rank. Why I was in the groove for that one and not the other, I can't say. Near the end, I got a lot of E or D ranks to scrape by and continue the story. The only one I struggled mightly with was Rhythm Game 30 when you hang glide. I couldn't understand it and it was the biggest roadblock to my continuing the game. I finally squeaked through it with an E rank and could keep going. When I went back to get A ranks on everything, it was the second last game I did. But playing it over and over I finally clued in to how it worked and when I had that moment of clarity, I finally appreciated it and it removed a lot of the negativity I had around it. I do wish the game sometimes explained things a bit more because it took me so long to grasp that concept and it only appears the one time.
Another thing that helped was the items you can buy before beginning any Rhythm game. The item to replensh my groove bar if I reach 0 helped me get through a few tough challenges in the end to keep moving. Then when I was working on getting the A ranks, the item to stay in the groove helped me so that I wasn't as heavily penalized by a mistake thereby helping keep me in the running for an A rank at the end. That said, it is true you have to be pretty much perfect at the end of any Rhythm game as you won't have time to rebuild your groove energy if you make a mistake then. But at least it helps you through the first two parts and a bit of the start of the third.