I reckon Autronic are best bang for buck and reliable. We fitted SM2 to that black Harrison my brother now races. I also noticed that DRB also fit Autronic to all their turnkey GT40's, they use SM4.
We initially thought Motec, great product, very flexible, but the cost kills you. And forget about optional extras like traction control, launch control, boost control, data logging etc, you would need to morgage the house.
Watch out for hidden extras. Autronic come standard with wiring harness, map sensor, data logging etc, all stuff that costs extra with Motec.
I actually run an old EMS in my GT40.
Yeah, lots of options, good luck.
Also whatever ECU you go with, ask for a startup map for your engine configuration.