Short answers: ecc vs. non-ecc is about the reliability of your RAM and whether or not you mind cosmic rays (and other more likely things like normal component degradation) having a chance of crashing a program or your system.
buffered vs non-buffered is a question of how many sticks you want to be able to stick on a motherboard. The buffering lowers the electrical load on the memory controller making it possible to put many more sticks on the same bus. Desktop board - no buffering needed, server/workstation boards - buffering needed for more than X sticks. X to be determined by the manuf. of said board/chipset.