hc4361
The clock speed of the 9.7" iPad Pro is 2.16GHz and the larger model is
2.26GHz. This means that in real world performance (opening and closing apps, multitasking, summoning Siri etc), you won't seen much difference between the two machines.
The reason why the larger iPad has more memory is that it has double the amount of pixels to push around (the 12.9" model is exactly the same size as two 9.7' iPads sitting next to each other).
And you can't add RAM to any iPad.