Regarding WP Starter and Bedrock: the latter is more a “boilerplate” for your project, the previous is more a library that you require in your project. It is a different approach, and one is not necessarily better or worse than the other.
As a boilerplate Bedrock is more opinionated on several decisions, e.g. folder structure or some libraries to use (see the list of Bedrock “require” https://github.com/roots/bedrock/blob/master/composer.json#L39-L42).
Being a boilerplate, you need to clone Bedrock and edit its source if you really want to customize something (but probably you loose one of biggest pros of a boilerplate: a standardized setup).
If you use WP Starter, you require it and use its configuration if you want to change something, like folder structure, and in case you want to use Bedrock/Roots libraries… (or anything else) you just require them.
If we consider WP Starter 2, beside of this difference there was no much more to say. Maybe that WP Starter 2 has better support for dropins and that Bedrock has a better support to use regular plugin as MU plugin.
Being a boilerplate, getting started in Bedrock is easier than WP Starter 2, because in the latter you had to configure your Composer “scripts” manually, in Bedrock you get them already when you clone the project.
With WP Starter 3, which will be released in 2019, and the conversion to a Composer plugin, this latter point is not true anymore: as written in the article being a Composer plugin allows to run scripts without them being configured by end user on project composer.json.
The end goal of Bedrock is to have a standardized (and so, opinionated) way to use WP + Composer + ENV + a set of libraries.
The end goal of WP Starter is to provide tools to build a different WP + Composer + ENV setup per project, based on project needs and requirements, or why not, build a standardized boilerplate to reuse (company-wide, team-wide…), if wanted.
In theory, people at Roots could use WP Starter 3 to build Bedrock: they would require it, their additional libraries and would ship the WP Starter configuration that would setup the site (starting from folder structure) like Bedrock do it now.
Would they find any value in doing that? Well, this is not a question for me 
I can say that if they would express the will to do so, I could offer my effort to make this as smooth as possible.
With WP Starter v3 some additional technical differences are there e.g. the usage of Symfony Dotenv component to parse .env files instead of vlucas/phpdotenv that is used in Bedrock (and was used in v2) and minimum PHP version that, as of now, is PHP 7.0 for WP Starter v3 and 5.6 for Bedrock.
Moreover, WP Starter v3 explicitly supports packages of type “WP Starter Extension”, so you can expect that in future some extensions will be created to enable more features on WP Starter, but always in a way that is end-users call to add them, if they wish. Spoiler: the first of them will handle installation of translation files for plugin and themes.