I've installed a few of the Seagull units and also the reverse osmosis systems.  I can't comment on the water quality from either but I assume it is good (I've drunk it!)

The significant difference is that the reverse osmosis units (such as the ones sold in Bauhaus and some ferreterias) work very slowly and have a small (5 or so) litre reservor and are only really meant for drinking water. They have their own tap and are usually fitted under (or near) the kitchen sink. 

The Seagull units are designed to supply water to the whole house and are connected in the main water pipe from the cisterna.  You will barely (if at all) notice any difference in the flow of water from the taps so water in any part of the house is safe for drinking/showering.

The Seagull units are installed with 3 normal filters of reducing particle size before the main filter.  These filters are only to remove particles from the water and although aren't essential help to prolong the life of the main filter in the Seagull unit.

Nigel