I've heard they are very much alike but I've not been down south to the beaches so I couldn't swear to it. The North Beaches are, of course, easier and quicker to get to. There are rumours of taxi problems down south too.
Ashvem Beach (between Morjim and Mandorim) is by far the quietest that I've been to. Good fishing there too. The beachis much shallower too so the waves break much further out than the likes of Candolim meaning you don;t get wiped out having a paddle.
There are en-suite huts there that'll cost about 400 - 500 rupees per night per hut (sleep 2). You get a double bed, mossy net, shower and toilet. Try Seaview or Rock Bites, both very good with very good food but watch the prices of the seafood. Make sure you negotiate the price before you agree to eat it.
Personally I find Arambol a little crowded now. Not that its teeming with people but it is busier than Ashvem. The hippies are quite amusing though and the Ban Yan tree at Arambol is well worth a visit - trust me. Also there is therapeutic (alledegly) mud at Arambol which is great fun if nothing else.