Which one do you rather use? Police state, universal suffrage or representation?
It depends on what you are trying to achieve.
And, generically speaking, it is quite straight forward:
- PS if most of the hammers are invested in military units AND/OR you have WW problems (which of course means that you are at war); it might be good also planning a war, of course.
- REP works well in SE and to speed up your research if you have libraries (and/or similar buildings) and/or you are running caste system; by the way, under REP mercantilism gain some strength and might also be kind of more useful than Free Market in some situations; and last note, if Rep comes from the 'mids, some other early wonders (as Great Library and Parthenon) or NW (as Oxford or NE) in a "GP farm / wonder spam" kind of Capital also works well together with Rep, boosting its efficacy. In the end game (modern era) the economy under Rep goes often a little in difficulty.
- US AFAIK is great if (usually in the end game, of course) you have a big empire not too much war oriented, with a very big and complex economy, likely with corporations, and a good number of towns in your CFCs, which is a rather rare situation for myself that either go for a domination or (more likely) I don't expand too much when aiming a peaceful victory. So I can't really say at its best how good it is. I use it with Free Speech, Free Market and Free Religion in a CE oriented game, when I play focusing strongly on cottage spam, avoiding SE and wonders plans. I (speaking for myself) like this kind of game strategies a bit less than Rep and SE, especially if my empire is small enough to leverage Rep at its best.
However, I'm not in the master players on this forum, so you'll likely get better points of views from other posters. I just wished to drop my 2 cents in this discussion.
Cheers.

- yatta