i think that based only on the transfer that happened so far we cannot generalize and say la liga will be the strongest. i mean apart from samuel and maybe fiore , i don't see the coming players to la liga are such big names (larsson is not at his peak anymore).
even if the transfer season ends this way (which i don't see- still coming players to serie a) i could also say that still a generalization of la liga being superior is not correct. don't forget that italy, as a very strong national team, in itself has many italian star players if compared to spain. so serie a is also getting star players from within, and not always imports: cassano, gilardino,pirlo, etc...
another observation from my watching of serie a is that of the big names that rise in that league came when they were no that famous: van basten, shevshenko, kaka. which is a little bit different than is spain where they are after big signings:zidane, ronaldo, beckham, etc... (Exeption: ronaldinho

)
this is my personal opinion and of course i could be wrong