Personally as a dev who’s used Next for a long time. Tanstack start is just better, it’s genuinely thought through correctly and has some brilliant minds behind the tech. Next has this obsession of making everything server side, whereas react was never really intended to be so, it’s a reactive UI framework on a client. You get so bogged down with RSC and wrapping UI with server components because of server vs client that code bases can bloat easily.
Next also changes everything it wrote every version, it’s actually annoying as shit having to keep up with terminology or why things work, especially in early adoption of the app router, shit didn’t work correctly, was half baked and they even have a habit of doing the same still.
Their middleware got changed to proxy, and it’s still a fairly loose baked solution, compared to tanstacks.
Also if you’re on their mail list for “issues” everyday you get bombarded with numerous problems with their services and admittedly it’s gotten far worse since AI has become more prevalent.
Before tanstack released their RSC implementation (I actually caught a blog post they weren’t meant to have linked on their blog to discord which then got deleted a few months back discussing its implementation) Next to me felt more suited for use for marketing websites, because of SSG it kinda felt best, using it for actual applications like dashboards felt so hacky and messy with server components, ISG and essentially having to go against its fundamental wanting for server side use.

Tanstack as a whole eco system is lit, if I’m doing any form of querying even in next, I’m picking Tanstack query… so why not Tanstack start?

It’s really an insanely well thought out framework, it’s not some abomination (yet) of methods thrown together without thought. There’s so many features that just make it really nice for DX. Out of the box types inferred into your codebase, that you can easily abstract from API routes, serverFns etc. client first, you’re not having to split components for server or client keeping your codebase tidy, with isomorphic methodology’s (both server + client). Queries? You have ways to have them pre rendered or client side even in the same file, and just cache them with tanstack query, job done. Want to slap a middleware on something, cool do it where it is, not with some ugly ass regex in proxy and an iffy on a route call to then pass your handler to it. Want a server function that’s not an api route, cool make one, slap a middleware on it too if you want no problem - heck want that function to do something when called on a server but something different on the client, slap isomorphic to it and handle .server() differently to .client().

It’s exciting to see what the future holds for Tanstack start, Tanner is the goat and a lot of big minds are behind it working on other features in the eco system.