The two bullets I would recommend are the 250-grain Barnes TTSX and the 286 Nosler Partition. Haven't taken Cape buffalo with either, but when I hunted buffalo in Tanzania in 2011, my PH was so impressed with the way the 286 Partition worked on heavier plains game that he wanted me to use it on my second buffalo. (The first was taken with my .416 Rigby.) Both of us wanted me to take a bigger buffalo than my first--which was very good--but unfortunately we never found a bigger one. We did get up on a big bull that had one broken horn, and in hindsight I should have taken it, but....
Should also mention that though technically the 9.3x62 isn't legal for buffalo in Tanzania, the government game scout that accompanies safaris agreed to let me use it, as he had also seen what it did. African caliber requirements are somewhat flexible, due to various factors.
In my tests both on various kinds of media and big game, the 286 Partition slightly out-penetrates the 250 TTSX, but the difference is slight. In your 20-inch barrel I'd be tempted to use the 250 TTSX, which modern--pressure handloads should result enough velocity to expand well. On the other hand, the 286 Partition's soft front core will definitely expand even when started at somewhat lower velocities.
