If you really can push the car near the limits, I wouldn't do one without the other. If you're like most of us, you'll never actually test the aerodynamics, so just do what looks good to your eye.
Oh, and yes, I have one, and I will put on canards.
I'd agree with earlier posts, unless you are exploring the absolute limits with huge performance then aero tweaks are low down the priority list. But if you want a complicated project then ...........
Cheers
Roger Allen
I put a spoiler on the rear of my 1968 No 11 replica. I also put 'pockets' into the bodywork at the ends for authenticity. The fixings have subtle led's in them which have allowed me to get rid of the un-authentic looking reverse lights.