Montevideo Or Santiago?
 

I'm going to Buenos Aires and have the option of stopping in Santiago or Montevideo on the way or way back. Which city is better for a couple of days?

I just returned from a month long trip in South America.
I went to Sao Paulo, Santiago, Mendoza, Buenos Aires and Montevideo.

Each city was great but, Montevideo was the least exciting, it's sort of sleepy.
You have pockets of interesting things but a lot of dull space in between.
it's a great place to go to relax, enjoy the beach, but it was a bit too quite for me.
Everyone goes to Punta, it's their version of Miami Beach. It's about an hour and a half away.

Santiago is a vibrant, urban, sophisticated city, cheap, easy to use subway, very walkable, the wines are wonderful and lots outstanding restaurants.

Out of all the cities I visited Buenos Aires hands down was my favorite, awesome city. Santiago was wonderful, Mendoza was a huge surprise, a must visit for any outdoor enthusiast, wine and food lover! Montevideo was beautiful, laid back and relaxing... but at times a bit too quite for me. I spent 5 days there. Sao Paulo is a massive ugly city like Los Angeles and Houston.

Definately Santiago

More to do in Santiago.

Montevideo feels a bit like a town, Santiago like a proper city. I like cities, so I'd go for Santiago.

How many days in Santiago? I have 6 total for it and Buenos Aires.

you have 6 total days to visit both Santiago and Buenos Aires? I'd spend most of my time in Buenos Aires, it's a larger more vibrant city. Santiago is like Denver or Sacramento.... nice cities but not a city tourist go out of the way to spend time in.

I spent 7 days in Santiago which was way too long, I was ready to leave, spent 10 days in Buenos Aires and did not want to leave.

I can't comment on Buenos Aires, but recently spent two full days in Santiago coming back from Easter Island. Two days felt just about right for Santiago, IMHO. Nice city, easy to walk and otherwise get around, great food and drink, and some interesting sights. But I didn't feel like I missed out on any "must-sees" after a couple of days.

Now, there are things to do in the region - that I wouldn't mind going back to do in the future (although I'm interested in checking out some other areas in Chile first), but as for the city itself it was a good length of time for the wife and I, anyway.

Santiago over Montevideo, definitely.

With only six days total, I'd go for one or two in Santiago, the rest in BsAs. Maybe one day in Santiago, one in Valparaiso.

Better for what, exactly?

As others have said, Santiago is a modern, cosmopolitan city. Montivideo is more of a cow town. But it's also cheaper than Santiago for most things. For me personally, I would spend 2-3 days wine touring in the area outside of Santiago.