If you had the choice between going to Ushuaia or Punta del este which would you choose? Which place has more to see and do?
I am going in March.

Your choice is an apple or an orange. Do you want to go to a beach resort in a warm climate or to a less developed, colder part of the world in Patagonia?

My choice is irrelevent. It is what floats your boat that is important.

Weather is irrelevant to me. I am more interested in sightseeing.

Patagonia offers more by way of sightseeing. Punte del Este is a beach resort, albeit a very nice one!

I guess drdawggy gave you a very good reply, two totally different things, Punta del Este is beautiful as a resort, as a beach resort goes. Lots of pine trees and several different kind of beaches. You can go sailing, watch the sunset,go fishing , dancing.
We are talking of one place.
I have never been in Ushuaia but as far as I know it is deep south near Antartida , brrrr...cold and you can indulge yourself in endless cold panoramas. The Patagonia is a huge extension with beautiful lakes , mountains, and forests.
I should say that it is senseless to compare things that are so different it is like asking, to choose between
Positano and Finland...