I rate Bale as a player, he is definitely a match winner.
That said, I really feel that there are 2 factors that he has at Tottenham that he wouldn't get at say Madrid.
1. His given basically a completely free role to move around and contribute how he wants. If he moves to Man City, Madrid, United or us he's likely to find his role to be a little more structured. This isn't such a huge issue in the sense he is originally a left wing back and can play a defensive role if required and this can actually be an advantage for him as he's more likely to be able to fit in a big team if his not only able to play a completely non-defensive role. But it will mean he creates less than he does currently at Tottenham.
2. This is mostly about the league. But while it's true smaller players can absolutely be productive in the Premiership with the whole needing to be big thing a bit of a myth, being quick is actually pretty important.
Shevchenko for example struggled at Chelsea not because of lack of skill, but because he wasn't quick enough and the game tended to "pass him by" at times.
Ballack when he first joined Chelsea was played at the tip of the midfield diamond by Mourinho and similarly struggled to keep up until Grant moved him into a deeper role alongside Lampard where he played the rest of his Chelsea days before departing.
Anyways, while pace is important in the premiership, players who rely on pace like Thierry Henry (at Barca), sometimes struggle in slower leagues like La Liga because thr slower tempo allows opposition teams to get back and constrict the space better.
I can see Bale (like Henry before him) struggling to replicate his Tottenham form in another league.
TL/DR?
Basically i'm suggesting while a wonder player, he isn't likely to be AS good for another club as he is at Tottenham where everything is set up for him to succeed and if he does move he should definitely remain in England as it suits his game most.