Usually these different tartans signify different branches of the family. The purists want to wear the correct tartan for the branch of the family they trace their ancestry to but many do not worry and just pick whichever sett (colours) they like the best

Do you know your ancestry? If so, the answer lies there. If not, then pick whichever you prefer best. Before you do so though double check with the clan association web pages just to see if there are any protocols there that you want to be aware of (for example, the Campbell's do not officially recognise the Campbell of Argyll sett)