The advantages of the QC is size, soldering posts and punch adjustability.
There's an on going rumour about Quantums blowing up for no reason. However, there's a couple in our club, and they've never had any trouble.
The advantages of the 7.1 are better specs and it's wll proven.
Personally, I'm running the 7.1 in stock class. It needs a little more punch, but the problem wasnt bigger, than I could adjust it away on my TX. It has to be said, that I used the 7.1 ½ year, without making this adjustment.
If I had to buy a new speedo today, it would be one of these:
LC-Display - programmable read and set for all parameters
PC Interface - download of user defined Profiles (optional)
Mini Gold Connector - replaceable power wires without soldering
Single Button Setup - build in, optional external Power Switch/Single Button Setup
Voltage Input: 4-8 cells
On-Resistance: <0,001 Ohm
Recommended motor: > 9 Turns
B.E.C. Output Voltage: 6.0 V
B.E.C. Impulse current: 3.0 A
B.E.C. Constant current: 1.0 A
Case Size: 33x23x16mm
Weight: 22 g
The great thing about the CS, is it can change the drive frequency depending on how fast you move the throttle.