Both are correct, and they have practically the same meaning.

By themselves, "**pro**" and "**por**" have quite different meanings. 

Basically, "**pro**" means "*because of*" and "**por**" means "*for*". 

Pretty much all the meanings of English "*for*" can be expressed with "**por**", with the exception of the meaning of "*because of*": If you want to say "*for that reason*", you have to say "**pro tiu kialo**", not "**por tiu kialo**".

Zamenhof used both "pro" and "por" after "danki", with no recognizable difference in meaning. Both prepositions can be justified after "danki": Using "pro" makes sense, because in a sense what you are thanking for is the cause of your thanking. Using "por" makes sense despite the fact that "por" cannot mean "because of", as "por" can mean something like "in exchange for": You can say "Mi pagis por la biero" ("I payed for the beer"), and analogously you can say "Mi dankis por la biero".

So both options are equally good, and they mean the same. Which one you choose to use is just a question of personal taste.