As a spanish guy with an argentinian girlfriend, I couldn't recommend one country above the other...

...but, seriously, if you want to learn and study a more accurate pronunciation and vocabulary, you better spend the year in Spain. I don't mean that the argentinians don't speak a nice castillian! (and they speak with a nice accent) but their pronunciation, vocabulary and accent are very influenced by the italian presence in the country.

Spanish is just one language, but it changes noticeably from one country to another. Of course, every national variations of spanish are all right to me (some of them are very beautiful), but, historically, the classic spanish is better preserved in Spain (specially Castilla, and parts of northern Spain. Even in Valencia city the pronunciation is very "classic" or "standard", but usually not in the sorrounding valencian-speaking pueblos.

In latin-american countries, for example (as in parts of Andalucia) the "z-" and "ce", "ci" are pronounced as an "s-". The original pronunciation remains only in Spain. In Argentina, they also make sound the spanish consonantic "y-" and "ll-" as a kind of "shh-" (and, you know, that's beautiful), although they study the original pronunciation at school.

In Argentina (and Uruguay) there also remains a deformed ancient use: they speak using "vos" instead "usted" or "tú".

In past centuries, one person could speak to other using a respectful second person plural (like "vos habláis muy bien" instead "tú hablas muy bien"; and "vos parecéis cansado" instead "tú pareces cansado"). In argentina it remained as "vos hablás muy bien", "vos parecés cansado", etc. This form is not in use in Spain, Mexico, Cuba, and the most spanish speaking countries.

Of course, in Argentina you will study a standard spanish, but the differences between the standard language and the everyday people language are much more in Argentina that in Spain.

And now, I've got to leave running.. my girlfriend will want to kill me!