Rio or Buenos Aires?

I have the chance to travel for 2 months and I have these two options:

-Rio from NYE until Carnival for 8 weeks -Rio 3 weeks during carnival + 5 weeks in Buenos Aires in March

Which one do you think would be the best option? Considering overall fun, activities, livability and vibe.

I don’t need to necessarily visit two countries. My question is mostly regarding which option would make me enjoy myself the most and have the most fun overall