It all depends on how far from the hospitals (and which hospitals and whether you have medical insurance) you are. I don't know the cape peninsula that well, but I would say that Panorama or Milnerton mediclinic in the northern regions are pretty much the best out there, and perhaps Chris Barnard in the city centre, or Southern Cross in the south. Most of the paramedics (of which I am one) tend to refer their most critical patients to these hospitals, but they are all private. As far as government goes, I would say either GF jooste on the "flats" (yes I know, but I know a lot of the docs there and they know their stuff), or Tygerberg or Groote Schuur. for kids, Red cross is the place to go.
As far as keeping antivenom, the last 9 amps that were used on me in June with my mamba bite cost in the region of R700/amp. They only started working after 5 1/2 amps had gone in... that's a lot of money to keep in your fridge that you may never use. With Cape cobra's you have about 6 hours before the effects of the venom do not respond to antivenom therapy and you have to wait them out (as quoted by Branch), and the boomslang venom is a fairly slow acting one, and you should have more than enough time in both cases unless there are significant extenuating circumstances, to get to a suitable medical facility. As far as I know Antivenom is kept at both Groote Schuur and Tygerberg, I can confirm this if you want, but either way it is available in the cape peninsula and the Red Cross Air mercy services will FLY it out with a heli if it is urgently needed.
If you do some searching ont he site, you will see that there was recently a green mamba bite in the cape region that was managed without AV at one of the hospitals, there was a Cape cobra bite in the oudtshoorn region that was managed succesfully with AV at a hospital that I would not have thought would have managed such a case as well as they did, and there was my bite which I managed in West Africa. In my opinion, unless you keep a ton of really venomous snakes, and have a massive risk of getting bitten, the money you'd spend on the AV, would be better spent on doing a snake handling course and getting a good hook stick or grab stick so you can safely chuck them back over the wall. It'll cost you way less and make you more aware of what you are dealing with, without the fear of something going wrong.
Lastly Tygerberg poison centre is pretty top notch when it comes to giving out advice, when you can get them to answer the phones
hope that answers your questions...