They both have topographical features that lend themselves to absolutely stunning photography (patently obvious, but I had to say it). In Rio, ascending both Pão de Açúcar and Corcovado makes for a landscape photographer's feast (plus, at Pão de Açúcar you could watch the planes going to/fro Santos Dumont Airport).

For day trips, Cape Town has the Cape of Good Hope, Stellenbosch, and wineries, while Rio has its own rainforest, and Niteroi 's contemporary art museum (less for the museum than for the architecture).

I'd return to both, however, I was accosted a couple of times in each place (not quite successful in getting my money, but aggressive and a pain nonetheless); in Cape Town, it was on Long St., and Rio, downtown by the Petrobras HQ.