Can someone please recommend a bus company for travel from Santiago to Mendoza, and then from Mendoza to Buenos Aires?

I'll be flying into SCL in early October. Will stay there for 2 nights, then take an overnight bus to Mendoza and stay there for 2 nights. Then take another overnight bus to Buenos Aires and stay there for 4 nights. Since it is kind of a long journey, i'm trying to find an affordable/comfortable/reputable bus company to make the journey with. I'll be on a tight schedule so i would prefer to find a company that i can book online with to have a guaranteed seat.