Defenitely go with the cdi. I am currently running the 170cdi in my 2 Valiants. I have 175 cdi's ready to go in when i get time. With the cdi my idle is about 1600 rpm. With glow plug it is about 2300rpm.
Fuel usage would be about 1/2 to 5/8 compared to glow (depending on wind).
I use 30% nitro, 10% low viscosity Coolpower oil and 1% castor oil. You need to keep the plug gap on the tight side to prevent the engine from misfiring.
You also need to be carefull when adjusting the needle, as you get very little warning if you get to lean. On 30% nitro my needle is approximately 5/8 of a turn out.
I have run mine with a glow plug without any trouble. Top end rpm seems to be similar, but the cdi gives much better mid range power. Don't need to use full throttle. I also had to richen the mixture to 1 1/2 turns out.
For the ignition i am using the Aero Tech IBEC plugged directly into the Rx. Works brilliantly, no interference at all.

Scott