Hart would be an upgrade but it's not worth it for his price -- especially if it's just a loan. He's better than average, but his wages/price tag are greatly inflated.
As far as which leagues are better... Spain and Germany have some better teams (Barca, both Madrids; Bayern, BVB), but England has the most parity by far. Teams in other leagues can afford to rotate their squads regularly and put more focus on European competition.
I'd be curious to see how injuries come into play since the English League is far more physical.