Stipends are entirely dependent on the institution. There are some nuances, but in general every trainee is on the same pay scale for their given year - so the 2nd year PICU fellow (PGY5) makes the same as the PGY5 General surgery or neurosurg resident. This ignores moonlighting, which in some fields (EM notably) can be substantial, if allowed. The payscales usually see incremental increases from year to year such that a PGY 1 starting in July of 2019 will likely make a little bit more than the PGY 1's that are starting 7 weeks from now, probably by about $1000 in many spots.
Stipend differences between institutions tend to be very much based on cost of living for an area. High COL locations like SF, DC, NYC and Chicago pay more than Little Rock or Omaha. In some locations (DC and NYC), the trainees are unionized and as such have commanded a higher salary (though because of the increased cost of living it's not like you're banking). In cities with multiple training institutions, eg Chicago with NW, Rush, UofC, etc, the stipends usually are in the same ballpark of each other to the point you probably would have other things that separate them out on your rank list.
Personally, going from a low COL location for residency to one in a high COL city, the higher stipend in fellowship ended up being worth less money - the city was a lot more fun, I was going out a lot more frequently and the rent was significantly more.
