I’ve done both although neither with DC.
Costa Rica is my favourite country that I’ve been too. The first thing I’d say is you mostly go for the micro-fauna. If you are interested in birds, reptiles, frogs and insects then it’s heaven, although the monkeys, sloths and tapir are quite easy to spot too (and adorable). We didn’t see any big cats, but some people we met did. If you want to be shown big, impressive animals with minimal effort it won’t be the country for you - most tours set off at the crack of dawn or at night time and require lots of patience! It’s accessible when you are there (we drove ourselves from place to place, but organised transfers are plentiful) and the people are genuinely very friendly. The food wasn’t stunning - I’m not sure you go to central or South America for a gastronomic tour though. There are hundreds of little microclimates across Costa Rica, but generally august will be a bit more wet than average. The beaches on both sides are simply phenomenal. The main touristy areas are north, around Arenal volcano/cloud forest and Manual Antonio national park. A trip to the south of the country really is worth it though, if time permits.
We tagged on a safari to a trip to Cape Town a few years ago. It was in a lovely little reserve near Port Elizabeth, called Amakhala. Alongside being small, it party appealed as it was malaria free. The animals were as incredible as you would image. For me, 4 days at the lodge were plenty. You obviously can’t just set off on your own into the reserve so you are quite tied to the lodge and their timings. The food was plentiful and beautifully cooked. It will be cooler in August, but again this will reduce numbers.
Flights to both will be expensive, but I suspect far more expensive to Costa Rica, if you want to go directly.
Hope that helps! Have fun planning!