Orangecrest,
the phosphoric isn't as strong in ionisation energy as acids such as hydrochloric, and sulphuric. Therefore you can let it dwell a little longer. Don't get me wrong it left on for long enough it will etch. You just have a lot more time up your sleeve with this one. Another bonus it that it is more user friendly to work with than HCL or H2SO4, but you still have to respect it.
Environmentaly, it is a good fertiliser, but not good to go down drains as it aids in eutrophicataion of waterways. Quite often you'll see detergents that are "Phosphate free " because of this.
I haven't tried the citric yet,I'll have to check this one out. I am a strong believer of let the right chemical do the work of cleaning and I just rinse off.