I can only speak for the Holley and Accel systems as those are the only ones I have personal experience with. From what I've read, FAST is one of your higher end systems. I know absolutely nothing about Edelbrock.
Holley Commander 950 Pro:
Low end no frills. Lower cost alternative to DFI. Programmed via laptop PC. Easy to understand and tune. Bank to bank injection. Wideband 02 compatible and can easily accept any manufacturers wideband 02 controller input. Forced induction/NOS compatible. Can control ignition timing using inexpensive, readily available late model small cap GM HEI distributor. Excellent documentation. Good tech support. There is an EFI forum on Chevytalk.org that is virtually all Holley related and a Holley employee who runs the C950 on his car responds quickly to tech questions. Oval port EFI intake manifold is soso. Needs a lot of porting to match oval port heads. Only two sizes of throttle body available. 1000cfm and 2000cfm. 2000cfm throttle body has dominator size flange.
Accel DFI Gen 7:
Things may have changed since I used this system. High quality hardware. Wiring harness is much nicer than Holley's. ECU seems very well built and solid. Nicer software user interface. Programmed via laptop PC. Wideband 02 compatible but was not as easy to adapt a non-Accel wideband 02 controller to it. They tried to make it hard or impossible so you were forced to buy their WB02 controller. I heard they were going to change that with a new software realease but I sold my system before it came out. Several different throttle body sizes and configurations available. 1000 cfm, 1200 cfm, 1350 cfm, 1550 cfm, etc. Oval port intake manifold is much nicer than Holley's. Can control timing but requires a special DFI only distributor. Can run sequential injection with that distributor also. Distributor requires an external ingnition box. Has an injector trim feature that lets you fine tune the pulse width of each injector individually. Very nice feature. Has two different "keys" or "tongles" that let you access different features of the software. The standard key gets you partial access to the software and the "Pro" key unlocks more features. I found this to be VERY annoying. If you lose the key, like I did, you're screwed. The system comes with the standard key. The Pro key is an extra cost option. Stupid. Tech support was next to non-existent. This was also very frustrating.
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