JonM, 3458A will run circles around 34470A for stability, noise performance and accuracy and best on market linearity.
34470A is bench meter (a good one), while 3458A is reference meter, so they are build and tested to very different purposes.
While the later can be used as bench meter, it's not what it is best for. That is a reason why 3458A designed late 80's is still manufactured and sold new today.