I like grumichama best, but you need to consider whether or not it will be a good fit for your scenario. It is drought sensitive and therefore needs supplemental irrigation during the dry season. The wood is tougher than steel and may not be easy to prune without a chainsaw once it gets older. For tall (15 foot), untrimmed hedges with supplemental irrigration, the grumichama is ideal -- it's almost like (clumping) bamboo. Also, some years fruit fly can infest the fruit.
Cherry rio grande is a mediocre fruit in my opin. It's also drought sensitive. My gigantic specimen met the chainsaw recently, since it would not produce without irrigation, and the fruit quality wasn't really worth fighting for. A mango tree replaced his spot :-).
If you can get a high quality surinam cherry, I would think that would be the ideal plant for a low hedge. I think they are more drought tolerant too.
