I assume you are referring to High or Low Density memory:
A lot of people can obtain high density memory for a cheap cost. These as usually branded as generic or do not have a brand name on them. While they save money on this memory, it may not work with their specific motherboard. A majority of motherboards do not accept high density modules. They usually show or register as half of what they spec out or advertised to be (i.e. bought a 1GB module and it only shows as 512MB), or they dont work at all. This is common on all size modules.
LOW DENSITY modules have 100% compatibility with ALL systems and ALL chipsets. HIGH DENSITY modules only have 10% compatibility and are VERY SLOW.
So, TBH Low density is the best way to go about...