Not sure what you mean by preformance. I can tell you that the PF40 is a very basic drive. It has an old style interface, red leds and arrow keys for programming. It can be set up on devicenet and ethernet. Has pretty much your standard functionality. The PF70 has a really nice green LCD display. It has about 200 prameters setable from the screen. If you get the enhanced model for ethernet you have acess to 600 prameters from the compactligix platform. I have 2 of the enhanced drives and there pretty cool.
With all that said there both still VFD'S they both will control a motor. It's about howmany buittons and whistles do you want, need, can afford.