With an CD type system (MSD) what is the prefered plug?

Or does it even mater?

I was talking with a buddy yesterday and he had new plugs in his possession, for his race car, and I noticed they were resistor.

I usually run non resistor in all my race stuff.

I've used both and never noticed any difference MSD 6AL, 7 and Grid.

Digital CD ignition? Supposedly the electromagnetic interference from nonresistor plugs will effect the ignition (edit-> or any digital device for that matter) but I have no personal experience with non-resistor plugs

If your running conventional resistor plug wires the plug type probably won't matter. If your running solid core metal wires then your ignition may not like them with non resistor plugs
I ran into that problem with a early MSD 7 race box before MSD came out with the later 7A,7C and 7AL boxes, that box would miss a little from 5400 RPM to 5600 RPM, which was in the RPM range those boxes switch from multi sparks to single sparks per firing stroke

Spiral wound wires. Is the resistance better to have to push up the voltage?

If so the whats the big deal about low resistance wires then you screw in a 5k ohm plug.

I ohmed a resistor plug and got 4.94k ohms a non resistor showed .3 ohms

Measuring from the top of the spark plug wire terminal to the other end of the ground strap should read infinite ohms, not low ohms 2nd thought is your measuring the conductor itself so those reading seem correct
Measuring the center conductor on spiral wound resistor plug wires, not the spiral wire it self , should read more than 300 Ohms, I think
the air gap on the spark plugs is what makes the coil voltage go up so high to jump that gap.

Yes I measured the center conductor.

So I guess the consensus is it doesn't matter.

Pretty much. As far as I can tell, MSD doesn't mention anything about using Resistor / Non-Resistor plugs but they do stress "NOT" to use solid core plug wires.
IIRC they mentioned it at least three times in the Grid installation booklet