todella is only "really" or "truly" at its first definition. but also todella is a synonym for the word "erittäin" (very, especially). it was told by the Finnish dictionary, but English and French dictionarys don't admit it. idk why
In this context the only difference is that "todella" is more literary and "tosi" is spoken language. So both can be used here.
However, "tosi" also means "true" or "real". In that case, "todella" and "tosi" are not synonyms of each other and cannot be used interchangeably.