I am attending a 4-week Spanish course soon. They offer a shared student apartment with one to four others and private room for USD 120/week.
However, I've been thinking about a dorm in a hostel for USD 70/week.
After a month a friend of mine is coming and we are going to share an apartment together.
Any thoughts? Which is better of Milhouse and Portal del Sur?

a vote for the hostel
in web sites like hostel world among others you can check rating
the milhouse has a reputation for being the party hostel
happy travels : )

Sorry....a vote for the apartment. But really depends how seriously you are planning to take your studies..... How do you envisage getting any work done out of school hours in a shared dorm? At least in an apartment you are in theory sharing with other "students". If you are worried about your social life... my 4 weeks at school was the most fun I had in all my travels, because of the people I met there.
PS: Never heard of Portal del Sur, but Millhouse is definitely billed as the party hostel (ref paragraph above!)

Portal del Sur would be fine. I like it there. I work when I travel and every morning there was a bunch of us that set up on the roof next to the TV room. We just all sat at tables and worked. The other people there were students studying.

Another vote for the hostel! But not one which reputation is the party hostel (I don't know Milhouse indeed, I'm just seeing paragraphs above).
I was in BA a few days lately and I stayed in a hostel that seems good for your intentions. It's Extremo Sur Hostel, small, relaxed and well-groomed. Rates are affordable and appropiate as regards services. I think the website address was www.eshostel.com.ar<BR>Cheers!

I vote for the Apartment!
I've studied abroad 3 times, and did a family, an apartment and a hostel. I think the apartment is the best way to go. You meet really great people (who are also studying, partying, touring) AND you can relax, knowing that nothing is going to be stolen by some crazy person. Apartments usually have more room and kitchens, which is great b/c when you meet locals you'll have somewhere to take them.
Personally I think meeting locals is one of the best parts about travel and studying a language. AND being able to have them over for an 'american' dinner is a great way to bond and learn about cultural differences. -- I say american dinner b/c I'm american - of course this would change depending on where your from.

I stayed for one month in a San Telmo Hostel named Garden House and turned out to be a great experience. The staff is very friendly, the place is neat, clean and cozy. The best to me is the location (1 block from subway line E and 2 blocks from subway line c, plus numerous buses to anywhere in the city and the outskirts too). Their dorms are 9 and 10usd, so it is quite inexpensive compared to the USD120 that will cost you to rent an apartment.
I'm now renting an apartment with 4 more people, but for me, since it was the first time I visited Bs As, being in a hostel first allowed me to meet nice people and to get used to the city.