We stayed at both this past July.

Go with The Saxon. Its entry level rooms are spacious (>60 sq. meters), whereas the FS seems to be on the small side in every room category relative to others in the chain. The Saxon's service is slightly more personalized and assigns a butler as well. F&B at the FS was deplorable - we had three meals there and all of them were gag-inducing... they should be embarrassed.

When it comes to style, it's a matter of taste. I found the FS to be more modern and contemporary compared to the more colonial look of The Saxon. As others have pointed out, however, the FS could be just about anywhere. The Saxon is unique.

As an aside.. given how much awesomeness South Africa offers, I'd spend the least amount of time as possible in Joburg and focus my time and attention elsewhere in the country. We visited the Apartheid Museum, the Cradle of Humankind and the Rhino & Lion Nature Reserve in one day. Apart from - perhaps - visiting Soweto, I can't imagine there being too many other worthwhile things to do there (compared to Cape Town, to be sure.)