Oh wow... there is no comparison. In all honesty, Joburg is the armpit of ZA... sorry to any ex-Joburgers - Im too partial to CTown.. Its heavily crime ridden and not a beautiful place (aside from the Jacaranda trees!).
Cape Town can be expensive and it can be cheap. Depends on what you want. You can do the expensive hotels for about $1000 a night or you can get some pretty decent ones (I used to put some guests up at the Courtyard in Observatory.... very central and a brilliant place... for about $150.00 a night incl bfast). Go onto Hotels.com.... you'll see a huge variation there. eating out and even doing touristy stuff is very cheap for a tourist because of the exchange rate so enjoy it..... Here you spend a few hundred QAR for a dinner for 2.... there, you'll spend about $40.00 at a nice restuarant with some wine.

Brin