I have seen several people start comparing players side by side, to see which team is stronger in which position. This happens before every big game.
The issue is, that football doesn't work like that in the slightest (at least with players who are relatively close in level, as is the case here). So much is dependent on form and - far more importantly - tactics and team chemistry, how teams set up. Even if England have better players generally than Spain (something which I seriously contest as I don't buy EPL hype) Spain are a far better TEAM tactically. Their midfield (where you control games) is much better, and they're clearly a much better team when it comes to positioning, movement, synergy, as not only this tournament but both Spanish and English football historically bear out.
So, in that sense, it matters very little if (to take one comparison) Harry Kane is a much better striker generally than Alvaro Morata (which I agree with as I like Kane as a player). Because so far, he hasn't shown that, and he appears to be a tactical disadvantage for England, whereas Morata is like a Firmino-lite enabling the two wide lads for Spain.
England have some wildcards off the bench, though. As someone supporting Spain, I'd be more worried if Watkins was starting over Kane. He has pace, mobility and the ability to stretch teams.