Neither is opinionated. They have some pretty uncontroversial (similar to other popular languages) typing on top of JS, and it's all opt-in. If there's something you don't like, you don't have to use it.
> and their impact on an established toolchain?
I don't know about Flow, but TypeScript is very easy to integrate into an existing chain. If you're using something like Gulp, there is a pure JS library that you can call to transpile your TS into JS. If you use Babel, you can simply insert the TS->JS transpilation before you call Babel.
TypeScript has an executable called tsc that you can use to transpile your TS whenever the file is changed. You can use that, and then your tool chain doesn't even have to change at all. (Automatic TypeScript transpilation is built into WebStorm and easy to add to VS Code.)
> I have no intention of changing half of my tooling just to be compatible with what fb thought would be a good idea, and even less so for ms.
In my limited experience, TS is much more polished than Flow is. The tooling is mature, debugging works well, and the language is rapidly improved. It's not just MS using it -- Google also uses it, and some other larger companies.
> A build step is required, I take it, but that's not a problem. Would I be able to use old libs and build tools?
Anything (and I mean anything) you do with JS, you can also do with TS. It's a superset of JS and compiles to readable, idiomatic JS.
> Are there properly working code formatters for both?
WebStorm has mature formatting for TS, but there's also tslint if you want some additional error-checking. WebStorm has first-class support for TS, including integration with tslint.
> What about editor support (Sublime)?
No idea about Sublime, but WebStorm support is absolutely fantastic. Excellent debugging, hints, linting, and completion. VS Code is great, but it lacks some of the more niche-y features of the IntelliJ platform. Some people might not care, and VS Code will do everything they want.
Neither is opinionated. They have some pretty uncontroversial (similar to other popular languages) typing on top of JS, and it's all opt-in. If there's something you don't like, you don't have to use it.
> and their impact on an established toolchain?
I don't know about Flow, but TypeScript is very easy to integrate into an existing chain. If you're using something like Gulp, there is a pure JS library that you can call to transpile your TS into JS. If you use Babel, you can simply insert the TS->JS transpilation before you call Babel.
TypeScript has an executable called tsc that you can use to transpile your TS whenever the file is changed. You can use that, and then your tool chain doesn't even have to change at all. (Automatic TypeScript transpilation is built into WebStorm and easy to add to VS Code.)
> I have no intention of changing half of my tooling just to be compatible with what fb thought would be a good idea, and even less so for ms.
In my limited experience, TS is much more polished than Flow is. The tooling is mature, debugging works well, and the language is rapidly improved. It's not just MS using it -- Google also uses it, and some other larger companies.
> A build step is required, I take it, but that's not a problem. Would I be able to use old libs and build tools?
Anything (and I mean anything) you do with JS, you can also do with TS. It's a superset of JS and compiles to readable, idiomatic JS.
> Are there properly working code formatters for both?
WebStorm has mature formatting for TS, but there's also tslint if you want some additional error-checking. WebStorm has first-class support for TS, including integration with tslint.
> What about editor support (Sublime)?
No idea about Sublime, but WebStorm support is absolutely fantastic. Excellent debugging, hints, linting, and completion. VS Code is great, but it lacks some of the more niche-y features of the IntelliJ platform. Some people might not care, and VS Code will do everything they want.