Drive shaft RPM vs MPH

[THensley]

I have added an Auto Meter 5291 between the speedo cable. I have added a Holley 5in screen with GPS .My Holley HP will convert that signal to RPM or MPH.
What would you choose? Car is a solid 11.5 but I have enough parts to turn the boost up and make it a 10.5 ( I think )
What is best for tuning and logging purpose?

 

[TexasT]

I would do the rpm as the gps should be able to do the mph.

 

[TType85]

I am logging MPH but RPM (drive shaft speed) would be good too, I am using the 12.3 dash I replaced the cable with this 16 Pulse Speedometer Sensor Adapter for GM Vehicles

It is nice to be able to see wheel spin in the logs. I let out right as I felt the spin. I am sure you could do this with RPM just as well if not better.