I read somewhere that there could be a difference.
As a general rule, neither. While some situations would take a look at the last 12 months and others the last 5 years, the analysis is based on the standard of living attained during the marriage. For many couples, the highest marital standard of living is obtained right around the time of separation, and so looking at income at time of separation makes sense. For others (those in sales or a market-dependent line of work, or those with historical periods of unemployment), the standard of living fluctuates greatly over the years. For those situations, looking at the historical trend of income (depending on the lenght of marriage, 5 years may be sufficient) makes more sense.