Like I said the PL and La Liga are fairly close in term of competitivity and there is something that kind of "prove" that point, it's the percentage of wins in the league, so I used Statto they have an all time table.
So since 1998:
PL: United 0.741; Chelsea 0.708; Arsenal 0.702; Liverpool 0.628; City 0.565: Leeds 0.550; Tottenham 0.549; Everton 0.525; Ipswich 0.481; Newcastle .479.
Liga: Barcelona 0.741; Real Madrid 0.733; Valencia 0.604; Atletico 0.568; Sevilla 0.553; Villarreal 0.542; Deportivo 0.531; Celta 0.503; Athletic Bilbao 0.499, Mallorca 0.490.
You can clearly see that the density is comparable, they have the same amount of strong and regular teams. The difference is that in Spain there is a gap between the third and the second team but you can also notice that the gap between the first and the second is bigger in England, for the rest it's fairly equal, the spanish teams are just a little bit more stable.