Before we even begin to appraoch the subject of which league is better, we're going to have to first define what 'better' actually means.
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Better is a subjective term. "Better in terms of width/points/etc" - this is objective.
IMO La liga is 'better' in terms of the quality of football. EPL is more competitive, due to the fact that the quality levels of the team is closer together. In La Liga, there are bigger gaps between 'The Best' and 'The very goods' and 'The Very Goods' and the 'goods'. For example, In EPL the big uns are City and United, next is Arsenal, Spurs and Chelsea. Compare that gap in quality, to Madrid/Barca and Valencia/Malaga.
I prefer LL, but I admit I find EPL so exciting.