Although, if i look at the last 5 years or so then i don't know what Pakistan has done in test cricket so if push comes to shove then I'll put them in Tier 2 and move SA up in the pecking order.
Apart from performance, I took other factors as well, such as popularity of cricket in that country and size of the market. Pakistan is a country of 230 million people and cricket is the most popular sport there. South Africa has a population of approx. 60 million and cricket is the 3rd or 4th most popular sport. And at this point, I would say that both the nations are performing at the same level. But the market size should push Pakistan ahead of RSA. Anyway, it is a promotion-relegation system, so after every 3 or 4 years, teams who can't perform will get relegated.