I've travelled all over Argentina and spent 3 months doing a tour down the east coast and then up through the west to Mendoza, then east back to BA. All by Bus and it was a long trip. I came back saying to myself that if I ever gave advise about going south I'd say.... fly. Do not take ruta 40. Do not travel south of Puerto Madryn in the east by Bus. There's little to see other than wasteland.
Having said that, I highly recommend the glaciers of Perito Moreno and the hiking at El Chaltén (sister town of El Calafate). Just look these up on the internet and you'll get lots of photos and reviews. El Chaltén was a bit of a torture but I think that's part of the charm. By the time you get to the top you want to die and then when you see the view you want to live again.
Do not go up ruta 40. Go back east by Bus and then north or better yet, fly. Ruta 40 was an expensive painful trip for me. Someday I'll look back and laugh but so far, that day hasn't come yet. I've never felt so much like cattle than on that trip. It's all tourists on the Bus by the way. No locals would take that Bus or at least, that's what Argentines have told me.
Bariloche is nice and extremely popular with Argentines, so if you want to see Argentine tourists, this is the next best place other than Mar de Plata. Buy some jam and chocolates. If you've ever been near the Gulf Islands in B.C., this town will remind you of that. At least it did for me. Lots of good Hostels to stay at but one of the few towns where you might have to make reservations. DO NOT go during the graduation time of young Argentines. Or any kind of holiday. Lots to see and do.
My favorite spot was Mendoza. I also like the small welsh town near Puerto Madryn (Trelew) where you can go have traditional tea and eat cakes until you want to vomit. They just keep asking if you want more and I'm too polite to say no. Read the history of the place before you go or it won't be special. Go early in the morning and get out by afternoon. A very unusual Argentine experience.
Seriously though.... if you have a bit of time an money, you could make a nice trip for yourself. If I did it again, I'd try set up a multiple one-way flight schedule stopping in following places. Puerto Madryn, El Calafate, Bariloche. Then Bus up to Mendoza and back east. If you've got more time or the flight doesn't work, I'd buy a round-trip flight between Puerto Madryn (or Viedma) and El Calafate. I liked Viedma by the way and also most of the upper east coast. I even like the deep south like Rio Gallegos because it's not like mainstream Argentina.
Regarding Ushuaia, I've got to ask you why you want to go there. Or maybe ask yourself in an objective way. That's what I did and decided not to bother. Of course I had help from people that had just been there and convinced me not to spend the time and money just so I could say I went there. If you do go, fly there since the Bus travels through Chile territory each way and service is expensive and limited.
I stayed at Hostels everywhere I went other than Viedma and Puerto Madryn, which doesn't seem to have any. Look up hostels on various sites like
www.hostelworld.com or
www.hostelz.com to check out what's available.