Not sure where you are hunting but my opinion is the 270 would work but the 300 would be my choice. I understand the 270 is flatter however in most of the buffalo terrain I have hunted with exception to Tanzania there was not much opportunity for plainsgame at longer distances during the hunt. If it is a one rifle safari and you are hunting say Masailand I could understand the argument for a 270 grain bullet. However if you are hunting the Zambezi Valley per say then I see no benefit to a flatter trajectory as finding a shot over 200 yards is tough.
Its not so much to do with trajectory as simply velocity. A 270 or 300 grain TSX will open up to the same diameter, and like any good soft they will both be sitting under the hide on the far side of your buffalo. You may as well hit them faster. Actually, if you really want to hit them harder, use an A-Frame or a bigger caliber or (My favorite) an A-frame in a bigger caliber.
After my 4th buffalo I quit using solids.
