The reason you're getting mixed advice is because everyone has his or her own experience. Sometimes bus trips are totally fine and others are, well, not.
Having bused through Chile, Argentina, Uruguay, Bolivia, and Peru, I can say hands down that Peru was the absolute worst. (It was also my least favorite country by far; the two probably correlate.)
That said, nothing particularly 'bad' happened: the buses were at least 2 hours late as a rule; they were in a general state of disrepair; the drivers actively drank and smoked; locals peed in bottles instead of using the provided bathrooms. But no one hassled me or tried to steal from me and I was a Spanish-speaking Yanqui woman traveling alone.
However, you do hear about people, especially in Peru, who have horrid experiences. I have a good friend whose bag was stolen from under her feet and another whose backpack was taken from the baggage storage under the bus. Then you hear about roadblocks and foreign (especially English-speaking) people getting kidnapped for ransom, but I blessedly have never know anyone to whom that happened.
A quick search on Plataforma10 shows no buses from BsAs to Lima this Wednesday, but 1 by El Rapido Internacional that leaves Thursday at 18:00 and arrives Sunday at 17:26 - semicama (aka uncomfortable for a trip longer than 4 hours), sin servicio - i.e. no food or drink. Yikes! But, only 380 pesos, almost exactly $100 one way. (The same bus leaves Friday too, but not Saturday.)
Meanwhile, Aerolinas Argentinas roundtrip is $580, Taca air is $540 and LAN goes as low as $500 (for US citizens). If I were you I'd pay the extra 1) to save time and 2) to save hassle. But those are my goals.
If your goals are 1) to save money and 2) to have an uncomfortable but fascinating experience, it's more likely that your bus trip will be fine than it is you'll get kidnapped. Just don't take anything very valuable with you and I second the earplugs recommendation. Also bring a blanket; it can get really, really cold on those damn buses.
