Boomben's appraisal is mostly accurate of the two different leagues. Irvine Pony has a rep for being more competitive. They are fairly well-organized and well-capitalized. Irvine Pony has the best fields as they get to play most games at Harvard Athletic Park. Irvine Little League is supposed to be a bit kinder, a bit gentler, maybe a bit more fun, etc.
That being said, I am assistant coaching an Irvine Pony Fall Ball team right now (Mite-Ball, kids 3.5-4 years old) and my experience has been excellent so far. Our head coach has years of experience with Irvine Pony and according to him, if you look at the kids playing skill positions in high school baseball locally, a much greater portion of them came out of Irvine Pony vs. Little League. Pony doesn't modify rules nearly as much as Little League so the kids get used to playing the real game earlier... Even after only three practices and one game so far, the skills of a number of the boys have greatly improved. We had a little 3.5 year old gobble up a grounder in practice yesterday and gun it to first base beating the runner by 10-15 feet.