Hey Warty. I have successfully hunted both the sable and the buffalo.
Hunting the sable on foot is a very difficult hunt. My 'round-the-fire discussions with PH's and concession holders in Zimbabwe and personal experience have shown the sable to be a relatively soft animal, and much smaller bodied than you probably think they are.
On the other hand, the buff hunt is tough. If you're in open country, it is straight forward enough; you look over a few animals, pan the stalk, ban, bang, and if you're a reasonably capable shot, if it doesn't go exactly to plan, you will have some time for yourself to try again and your PH to back you. But if you can find some mopani forest, lots of thorns, an impenetrable mesh of bush that bites, then you're in for a treat. You'll have trouble seeing an animal at 25 yards. Identifying bulls will be very difficult, and evaluating trophies is near impossible. After many encounters with buffalo in these conditions, I have learnt that you have 5 seconds from making visual contact to get a shot off - by then they have heard you moving in the crunchy mopani leaves on the floor. So you have to be committed to your shot and quick to let it go in just the right place. In close quarters, there's not much room for error and any second chance will be much more in-your-face than you want it.
I say go the buffalo first.