A mature plant of either cherry of the Rio Grande or Surinam cherry can really load up with a lot of fruit. COTRG has far less eugenol in it, so the flavor is less offensive to people who don't like that furniture-polish after taste.
There must be a lot of variation in COTRG, though, since ours is not very sour at all.
We do grow an excellent Surinam cherry -- the cultivar 'Zill Dark', which is a large, nearly black fruit, with very little eugenol, and can be quite sweet.