There are numerous complications to the question you ask. Essentially, you've still lots of basic physics to cover: now isn't the time to necessarily worry about what type of research you will eventually go in to.
Don't be thinking about the standards school offer in specific research areas: this doesn't correlate with the quality of the undergraduate, which is what you're looking for. Focus on picking the school that offers the right undergraduate programme for you, rather than any sort of reputation in research. Getting a good foundation is the start to a good career. You won't touch anything like string theory at all at undergraduate level.
It is also difficult to predict where research will be in a few years time. That said, if you're set on preparing yourself for something like string theory, make sure to take a lot of math courses: though you likely won't have any meaningful choices in math classes until you're a couple of years in, come back to us then and we'll let you know what to select.
Otherwise, jobs in something like string theory are few and far between. And, at least in the UK, the amount a researcher (post-doc or otherwise) makes is standardised, and doesn't really depend on the research area, bar a few exceptions.