By better I mean England have better players. You could argue that Wales are more of a team than England but even if that's true, it can take you so far. Yes, we don't have anyone like Bale but all over the pitch we are "better". Our GK is better, FBs are better. Ashley williams could slot in our central defence. In central midfield, Dier and Rooney are marginally better than the Welsh with Ramsey. Bale is better than the likes of Sterling in the wings but then kane is better than their striker.
But I agree it all depends on how they perform on the day!
What is a 'better' player though, one who will accept his place in a group, control his ego and do a job, or one who is truly skillfull and technically a class above but doesn't really know his place and thinks he is better than others and full of ego even though on his day he's very good? Both have merits and failings. As a player you're only as good as the moment you're in, in the end. It's that simple. If we're looking at it on paper, technically, statistically etc then yeah England are supposedly 'better' but it's marginal and comes down to how well the team comes together
Wales have a good core and true team spirit and positive passion. Don't see the same togetherness with England, too many egos and players fattened up on PL salaries. Sections in the fans are yobs, vindictive and spiteful which feeds into the tournament overall. Slovakia looked a resolute and solid, if unspectacular, side who pulled together even though they ended up with a loss, with England rather than that toughness it seems like there's always somebody to blame, too many individuals. But yeah with tournament footy it changes game to game, Slovakia could lose it in their next game. Russia looked the worst side in the group from the first round and if England folded to that despite dominating it doesn't bode well. But again, all could change in round 2

